SPY vs. SPYM (formerly SPLG): Two State Street S&P 500 ETFs, Compared in Depth

Educational content only. This article is not investment advice and is not a recommendation to buy, sell, or hold SPY, SPYM, or any other security. It compares factual, dated, and sourced characteristics of two funds that track the same index — it does not rank them, does not state or imply which is "better," and does not match either fund to any type of investor or goal.

SPY (SPDR S&P 500 ETF Trust) and SPYM (State Street SPDR Portfolio S&P 500 ETF) are both run by State Street Global Advisors, and both track the identical S&P 500 Index. If you've seen the ticker SPLG in older articles, brokerage statements, or search results, that's the same fund as SPYM: State Street renamed and re-tickered SPDR Portfolio S&P 500 ETF from SPLG to SPYM effective October 31, 2025 (MIAX Exchange Group — Corporate Action Alert: SPDR Portfolio S&P 500 ETF (SPLG) name and symbol change to SPYM (PDF)). It did not merge with another fund, and its underlying index exposure did not change — only the name and ticker did.

Because both funds hold, in effect, the same basket of S&P 500 companies, the interesting differences between them aren't in what they own — they're in how each fund is legally built, priced, and traded. This article goes deep on exactly those mechanics: legal structure, cost, dividend handling, trading characteristics, and size. It expands on the shorter side-by-side entry for these two funds in our companion piece, SPDR ETFs Compared, and builds on the structural background in SPDR ETFs Explained.

Same index, different wrapper

SPY and SPYM both track the S&P 500 Index. That means, in principle, an investor holding either fund has proportional exposure to the same roughly 500 large U.S. companies, weighted the same way, at any given moment. Neither fund's manager is picking stocks or deviating from the index by choice — both are designed to track it as closely as their respective structures allow.

Where they diverge is everything around that shared index exposure: the legal entity each fund is organized as, what that entity is and isn't allowed to do with incoming cash and portfolio securities, what each fund costs to hold, how each one trades on an exchange, and how large each fund's asset base is. Those differences are the subject of the rest of this article — and none of them is a statement about which fund has performed, or will perform, better. That question is addressed directly in its own section below.

The two funds at a glance

The table below is a snapshot of factual, dated characteristics, each drawn from State Street's own fund materials, SEC filings, or a third-party data provider, cited beneath the table. Every figure changes over time — expense ratios can be adjusted, yields move with both distributions and prices, and AUM moves daily with fund flows and market values. Verify current figures against each fund's own fact sheet or prospectus before relying on them. This is not a performance comparison and not a recommendation.

A few things worth reading carefully in that table before moving on: the expense-ratio gap (0.0945% versus 0.02%) and the AUM gap (SPY is roughly five times SPYM's size) are both real and current as of the dates cited — but neither one, by itself, says anything about which fund has tracked its shared index more closely, which is a separate question addressed in the structure section below.

Structure deep-dive: unit investment trust vs. open-end fund

This section describes legal-structure mechanics as documented in each fund's prospectus and in SEC and issuer materials on SPDR product structure. These are mechanical facts about what each entity is permitted to do — not a claim that one structure produces a better outcome than the other.

The single most consequential difference between SPY and SPYM isn't cost — it's the legal entity each one is built as.

SPY is organized as a unit investment trust (UIT). Its own prospectus is titled, in full, "A Unit Investment Trust" (SPDR S&P 500 ETF Trust Prospectus, hosted by Wilmington Trust). A UIT is an older, more rigid structure than the open-end funds most newer ETFs use, and SEC and issuer materials on SPDR product structure describe several specific consequences of that structure (SEC EDGAR — SPDR ETFs: Basics of Product Structure):

• Full replication, not sampling. A UIT is generally required to hold the complete underlying index rather than a representative sample or optimized subset of it. SPY holds all (or substantially all) S&P 500 constituents in index-tracking proportions, by design and by legal requirement, not as a matter of manager discretion.
• No internal dividend reinvestment — the "cash drag" mechanic. As the roughly 500 underlying companies pay dividends throughout each quarter, SPY's trust structure holds that incoming cash uninvested rather than continuously reinvesting it into more shares of the underlying stocks. The cash sits, non-productively from an index-tracking standpoint, until the trust makes its next quarterly distribution to shareholders. This is described factually here as a structural mechanic — a real, if generally small, source of tracking difference against the index — not as a claim about how much it has cost or will cost an investor in dollar terms, which depends on prevailing short-term rates, dividend timing, and other variables that change constantly.
• No securities lending. Some ETF structures generate modest supplemental income by lending portfolio securities to other market participants (commonly short sellers or other institutions) in exchange for a fee, which can partially offset a fund's costs. A UIT structure like SPY's does not do this.

SPYM is organized differently: as an open-end fund within the SPDR Series Trust, not a UIT (SEC EDGAR — SPDR Series Trust, Form 485BPOS). That structure is not bound by the three restrictions above. Concretely, that means an open-end fund like SPYM has the legal flexibility to reinvest dividend cash it receives before its own distribution date, and it has the legal flexibility to lend out portfolio securities for supplemental income — tools SPY's UIT structure does not have available to it. It's important to be precise about what that flexibility means and doesn't mean: having the legal ability to reinvest cash or lend securities is a structural capability, not a guarantee of a specific financial outcome, and this article does not report, compare, or estimate either fund's actual tracking performance against the index — see the dedicated section below on why that comparison is deliberately left out.

Both structures are registered under the Investment Company Act of 1940, and both are designed to track the same index as closely as each structure allows — the difference here is about how each fund is legally permitted to operate, not about which one is a "real" or "better" ETF.

Cost: what the expense-ratio gap actually means, arithmetically

The following is a description of how expense ratios accumulate mechanically over time. It is not a projection, estimate, or claim about either fund's actual investment return, which cannot be known in advance and depends on the performance of the underlying index — a factor entirely separate from either fund's cost.

SPY's gross expense ratio is 0.0945% per year; SPYM's is 0.02% per year — a gap of roughly 0.0745 percentage points (about 7.45 basis points) annually ^{3 4 6}. An expense ratio is deducted from a fund's assets to cover its own operating costs, so, all else being equal, a lower expense ratio is a smaller ongoing deduction relative to the fund's assets. That gap compounds arithmetically the longer a position is held, in the same way that any recurring percentage cost compounds over time — a fact about how costs accumulate, not a statement about what either fund's underlying index will actually do, which is unrelated to cost and cannot be predicted.

State Street's own comparison materials note both funds' cost figures directly and describe SPYM (and its predecessor ticker, SPLG) as sitting within the firm's "SPDR Portfolio" line, a lineup the issuer positions around lower ongoing cost (State Street Global Advisors — Comparing SPY and SPYM (formerly SPLG): Two State Street ETFs for S&P 500 Exposure). That framing is the issuer's own positioning of its product line — reported here as a fact about how State Street markets these funds, not as a suitability judgment about which cost level makes more sense for any individual, which depends on that person's own circumstances.

Liquidity and trading: a separate question from cost

The following describes market-structure characteristics — trading volume, bid-ask spreads, and options-market depth — as reported by State Street and third-party market-data sources. These are descriptions of how easily and cheaply shares or options can be bought and sold on an exchange; they are not a performance comparison and not a recommendation about trading either fund.

Cost and trading characteristics are two different things, and SPY and SPYM illustrate that clearly. SPY is among the most heavily traded securities in U.S. markets and supports an unusually deep listed-options market. State Street's own liquidity materials describe SPY's secondary-market depth — measured by trading volume, bid-ask spreads, and options-market activity — as enabling a wide range of execution strategies with implicit transaction-cost benefits, and state that the year-to-date average bid-ask spread of competing S&P 500 ETFs has run more than double SPY's own spread (State Street Global Advisors — SPY Liquidity: Flexibility to Navigate Any Market). On the options side, third-party market-data trackers report SPY as the most active U.S. ETF by options volume, with average daily options volume in the millions of contracts and a large share of total U.S. ETF options activity concentrated in SPY specifically (Macroption — Most Active ETFs by Options Volume).

A bid-ask spread is the gap between the price a buyer is currently willing to pay and the price a seller is currently willing to accept; it functions as a built-in, per-trade cost that is separate from a fund's stated expense ratio. A narrower spread generally means a smaller gap to cross on each individual trade, all else equal — a characteristic that can matter more to someone trading frequently or in large size than to someone making an occasional purchase, though how much it matters for any specific situation depends on that person's own trading pattern.

SPYM trades at meaningfully lower volume than SPY. Options are listed on SPYM — its options symbols transitioned from the SPLG/2SPLG codes to SPYM/2SPYM at the exchanges and at the Options Clearing Corporation when the ticker changed on October 31, 2025 (MIAX Exchange Group — Corporate Action Alert (Options Markets): SPDR Portfolio S&P 500 ETF (SPLG) will change its name and trading symbol to State Street SPDR Portfolio S&P 500 ETF (SPYM)) — but both SPYM's own share-trading volume and its options open interest run well below SPY's, consistent with SPY's outsized share of total ETF options activity noted above. State Street markets these two funds differently: its materials describe SPY around trading, hedging, and liquidity-sensitive use cases, and the SPDR Portfolio line (including SPYM) around lower-cost, longer-horizon core holding rather than active trading (State Street Global Advisors — Comparing SPY and SPYM (formerly SPLG): Two State Street ETFs for S&P 500 Exposure). That is the issuer's own framing of how each fund tends to be used — not a suitability recommendation from Zydhra, and not a statement that either use case is preferable; which trading characteristics matter for a given purpose depends on an individual's own activity and goals.

Distributions: same schedule, a structurally different path to the shareholder

Figures below reflect each fund's published distribution schedule and a trailing, dated yield figure. They describe historical or trailing payments only — not a projection, estimate, or guarantee of any future distribution. Distributions are declared by each fund's trustee or board and can rise, fall, or be skipped entirely.

Both funds distribute dividend income from their underlying holdings on a quarterly schedule, per State Street's own published distribution calendar (State Street Global Advisors — SPDR Dividend Distribution Schedule (PDF)). As of the dates cited above, SPY's own fact sheet reported an "Index Dividend Yield" of 1.12% (as of June 30, 2026) and SPYM's fund page reported 1.15% ^{3 6}. "Index Dividend Yield," as defined on State Street's own fact sheets, is a trailing 12-month measure of the underlying index's dividend payout relative to the fund's net asset value — it describes income already paid, not a forecast of future payments, and it is not the same measurement as total return.

The small gap between the two figures is worth explaining structurally rather than treated as a signal: it can reflect the cash-drag and securities-lending differences described in the structure section above, differences in the exact date each figure was calculated, and rounding or methodology differences between how each fact sheet computes the number — not a difference in the dividends actually paid by the roughly 500 underlying companies, which is the same for both funds since both track the same index. Neither figure predicts what either fund will distribute going forward.

Size and AUM: a neutral fact, and why SPYM's asset base moved sharply

Assets under management (AUM) reflects a combination of net investor flows and the market value of a fund's holdings at a point in time. It is not a performance measure and not a signal about fund quality — a fund's AUM can rise or fall for reasons unrelated to how its underlying holdings have performed.

SPY's reported AUM was roughly $784–793 billion in mid-July 2026, per third-party asset trackers ¹³. SPY was, for years, described as the largest ETF in the world by assets; Vanguard's S&P 500 ETF (VOO) passed SPY in total assets in February 2025, a shift Bloomberg attributed to fund flows and cost competition between funds tracking the same index — not to any difference in investment performance (Bloomberg — Vanguard's VOO Overtakes State Street's SPY as World's Biggest ETF).

SPYM's reported AUM was roughly $159.3 billion as of July 20, 2026, per State Street's own fund page ⁶ — substantially larger than the fund's asset base before its October 2025 rename from SPLG ⁸. One documented driver of that growth: State Street stated, in its own July 1, 2026 press release, that the U.S. Treasury selected SPYM as the exclusive default exchange-traded fund for a newly launched federal savings-account program known as "Trump Accounts" (State Street Bank and Trust Company (Investor Relations) — State Street Investment Management's SPYM Selected as Exclusive Default ETF for Trump Accounts, July 1, 2026) ¹⁴. That is reported here as a factual account of one reason the fund's asset base changed — it is not a statement about either fund's investment merits. Being selected as a government program's default fund is an administrative and cost-related decision made by the program's designers; it is not an endorsement of the fund's investment quality, not a suitability determination for any individual saver, and it does not make the fund risk-free, insured, or government-guaranteed — a fund's assets remain invested in the S&P 500 and subject to the same market risk as any other fund holding the same index, regardless of which program uses it as a default.

What this comparison does not include, and why

This article deliberately does not compare, rank, or discuss how SPY's and SPYM's share prices or total returns have moved historically, and it does not project how either might move going forward. That omission is intentional, for two reasons:

1. Past performance does not predict future results. A fund's historical price movement, even over a long period, is not a reliable indicator of what it will do next — a principle that applies with particular force here, since both funds track the same index and any historical gap between them would reflect small structural and timing factors (like the cash-drag and securities-lending mechanics described above), not a difference in investment strategy or stock selection.
2. A two-fund, head-to-head return comparison reads as a recommendation, even with disclaimers attached. Presenting "which one gained more" figures side by side tends to function as an implicit ranking regardless of surrounding caveats, because that is how readers naturally interpret a head-to-head performance table. Zydhra is a portfolio-tracking and analytics tool — not a broker-dealer and not a registered investment adviser — and providing that kind of implicit signal about which of two specific, named securities to prefer is exactly the kind of individualized guidance that registered investment advisers are regulated to provide under the Investment Advisers Act of 1940 and related state law. This article stays on the structural, cost, and mechanical side of that line on purpose.

What this article compares instead — legal structure, cost, dividend mechanics, trading characteristics, and size — are dated, sourced, factual characteristics that don't carry that same risk, because none of them implies which fund an individual should prefer.

Reading this comparison

SPY and SPYM hold, in effect, the same investment exposure through two differently built vehicles: a unit investment trust (SPY), which cannot reinvest dividend cash between distributions or lend securities but has significantly deeper trading volume and options-market activity; and an open-end fund (SPYM), which can do both and carries a lower stated expense ratio. Which structural or cost characteristics matter more for a given purpose — trading frequency, cost sensitivity over a long holding period, options-market access, or something else entirely — depends on an individual's own goals, time horizon, tax situation, and risk tolerance, and that is a question for that person, or a licensed financial professional working with them, to weigh. This article's aim is narrower: to make sure that weighing starts from an accurate, sourced picture of how these two specific funds are actually built, not from an assumption that "same index" means "the same fund in every other respect."

For the broader SPDR family this pair belongs to — including the sector funds, GLD, and DIA — see SPDR ('Spider') ETFs Explained and SPDR ETFs Compared.

Disclaimer: This article is for general educational and informational purposes only. It is not investment advice, and nothing in it should be construed as a recommendation to buy, sell, or hold SPY, SPYM, or any other security. This article does not compare or rank SPY's and SPYM's historical price performance or total returns and does not project future performance for either fund. Zydhra is a portfolio-tracking and analytics tool — it is not a broker-dealer and is not a registered investment adviser. Expense ratios, assets under management, yields, and fund structures discussed above change over time and were current as of the dates and sources cited; they may no longer be accurate by the time you read this. Past performance does not guarantee or predict future results. Before making any investment decision, review each fund's official prospectus in full and consider consulting a licensed financial professional.

Sources cited