Originally published on WeeBet on 2026-05-25. Reposted with canonical link.

The headline question of which platform "wins" on prediction markets used to have an obvious answer. Through most of 2024 and early 2025, Polymarket led on volume, depth, and cultural mindshare. That story has flipped.

Kalshi hit a record $4.13B in weekly notional volume in early May, up 8.5% week-over-week, while Polymarket fell 6.2% to $1.60B — giving Kalshi 72.1% of combined volume.

But "winning" depends entirely on what you are measuring and which category you trade.

The headline numbers have inverted

For three years, Polymarket defined the prediction market category. Then Kalshi's regulated U.S. footprint, a Robinhood distribution deal, and a sports product caught up — and surpassed it.

The sector posted $8.6 billion in taker volume during April 2026, with Kalshi overtaking Polymarket to claim the top spot. Kalshi posted $5.42B in April taker volume, surpassing Polymarket's $1.99B for the first time.

The monthly trend tells the cleaner story.

Kalshi pulled away with record April volume of $14.81B (+13.3% MoM) while Polymarket fell 14.8%, widening the gap to $5.8B.

That widening — not the single-month crossover — is what matters. The crossover could have been noise. Two consecutive months of divergence is a structural shift.

Open interest tells a similar but more nuanced story.

Prediction market open interest hit $1.11B on May 1, 2026, with Kalshi and Polymarket holding 98% of it. Kalshi held $630.7 million of that total while Polymarket carried $449.9 million.

Kalshi leads here too, but the gap is narrower than weekly volume would suggest — a clue that Polymarket positions still represent more capital tied up per dollar of churn.

24-hour volume snapshot: where the money actually sits

A point-in-time snapshot from mid-May illustrates the texture.

Kalshi's 24H rolling volume was $291.2M (+1.7%), with top categories sports at $234.3M, crypto at $27.6M, and politics at $2.1M. Top markets included UFC 328: Chimaev vs Strickland ($16.0M), Game 3 Oklahoma City at Los Angeles ($14.9M), and Royal Challengers Bengaluru vs Mumbai Indians ($11.7M).

Two things jump out. First, sports is doing nearly all the work on Kalshi — and it's broad sports, including cricket. Second, the politics number — $2.1M in a 24-hour window — is almost a rounding error against the $234M sports figure. Whatever Kalshi is in 2026, it is no longer primarily a political event-contracts exchange. It is a sports book wrapped in CFTC oversight.

Polymarket's composition looks fundamentally different.

Sports made up $313.7M (23.8%), Politics/Gov $163.4M (12.4%), Finance/Fed $7.3M (0.6%), and Other $147.5M (11.2%) of Polymarket's monthly category split.

Sports matters, but it is not the whole show. Politics, geopolitics, and crypto each carry real weight.

Sports: Kalshi's structural advantage is now obvious

The single most important fact in this comparison is that Kalshi figured out U.S. sports before Polymarket could.

Sports accounted for 89% of Kalshi's 2025 fee revenue at $235M, with the sports share exceeding 90% in the final four months of the year. The explosive growth was driven by NFL season, with September-November generating $138M in sports fee revenue alone.

The category breakdown from May confirms how lopsided this is:

Kalshi's core sports notional grew 5.7% week-over-week and, combined with Exotics, held 82.4% of cross-platform sports volume. The NBA playoffs were the dominant driver.

Where Polymarket still competes is at the tournament level.

Polymarket's top sports market was the 2026 FIFA World Cup, which dwarfed anything on the Kalshi sports slate. The contrast reflects how differently the two platforms' user bases engage with sports, Kalshi leading at the game and series level and Polymarket volume concentrating at the international tournament level.

That's a meaningful distinction.

World Cup markets were already showing $327.2 million in 30-day trading volume, with 99.3% coming from Polymarket.

A U.S. user looking to trade Thunder vs Lakers wants Kalshi. A global user trading England vs Brazil in June wants Polymarket. The platforms are partially substitutable, but they have sorted themselves into different sports niches via their respective regulatory and geographic footprints.

Kalshi's parlay product — Exotics — is the other under-discussed accelerant.

The combo/parlay product was the standout growth story at Kalshi, up 23.2% week-over-week to $511.6M in notional volume.

Polymarket has no equivalent. For a U.S. market conditioned by DraftKings and FanDuel to expect parlay products, this is a real moat.

Politics: Polymarket still owns the category

If sports is Kalshi's stronghold, politics is Polymarket's — and it isn't close.

Polymarket continues to dominate Kalshi on Politics by a wide margin, holding 92.6% of combined cross-platform political notional volume.

The composition is telling.

Polymarket's slate was anchored by the US-Iran peace deal, both 2028 presidential nomination markets, and the 2028 presidential election. Kalshi's politics activity has a more domestic makeup, driven by LA Mayor, California Governor, and House control in 2026 midterms, with no individual market approaching Polymarket's top politics volumes.

There is a structural reason for this split.

War-related contracts make up a huge share of Polymarket's political volume — a category that Kalshi does not offer. If this section is kept apart, then it materially changes the comparison.

Geopolitical contracts — Russia-Ukraine ceasefire timing, Israel-Hamas hostage deals, U.S. military action — concentrate on Polymarket because Kalshi simply won't list them under CFTC oversight. That's not a market preference, it's a regulatory boundary.

The implication for traders: anyone serious about geopolitical or long-horizon political positioning has effectively no choice. Polymarket is the venue, or there is no venue.

Crypto: a category in flux

Crypto is where the two platforms are most directly converging. Polymarket has long dominated short-dated crypto markets —

5-minute BTC/ETH markets generate $60M+ in daily volume but are dominated by bots reading Chainlink oracle cadence in milliseconds. Longer-duration markets (monthly, quarterly, annual) are where manual traders actually have a chance.

Polymarket still has the depth here.

The Crypto category hosts 311 markets covering a wide range of subjects.

But Kalshi is now competing on a different surface.

Kalshi has expanded beyond fixed-expiry event contracts into perpetual futures on cryptocurrencies — continuously-traded derivatives with no expiration date. Perpetual futures are among the highest-volume products in crypto trading globally.

That changes the framing. Kalshi isn't trying to win crypto prediction volume per se — it's trying to capture crypto derivatives volume under a regulated U.S. wrapper.

Kalshi says its crypto-related markets have grown rapidly in recent months, with volumes increasing nearly 10x.

Polymarket responded in kind:

on April 21, 2026, Polymarket launched perpetual futures — a new market type sitting alongside prediction markets.

Both platforms now want to be derivatives venues. The crypto category is becoming the testing ground for that pivot — and the lines between "prediction market" and "perpetual futures exchange" are blurring fast. Risk note: short-dated crypto markets carry concentrated bot and MEV risk, and on April 24-25 alone, MEV extractors pulled roughly $40M from Polymarket's 5-minute markets. Retail traders should size accordingly.

The fee paradox: who actually makes money

Volume is not revenue.

Polymarket collected $29.22M in April fees despite trailing Kalshi in volume, signaling higher-value contracts. Polymarket collected $29.22 million of the sector's $31.15M total. The fee figures indicate that Polymarket, despite trailing Kalshi in taker volume, continues to extract a disproportionate share of sector revenue.

This is the most important counter-narrative to the "Kalshi wins" headline. Polymarket users trade fewer dollars but pay more per dollar — because Polymarket's markets concentrate in higher-conviction, longer-duration positions where traders accept fees of up to 1.80% on crypto markets. Kalshi's sports volume churns at near-50/50 odds, where notional volume is inflated relative to economic stake.

Kalshi has acknowledged this discrepancy in its own data presentation.

Kalshi counts volume by multiplying contracts by their $1 face value rather than the price paid, and records both sides of every transaction separately. A contract bought and sold at $0.10 generates $2.00 in volume against $0.10 in actual dollars exchanged. The longer the odds, the wider the gap, and for the same reason headline volume figures bear little relationship to Kalshi's actual fee revenue.

This methodology distinction matters enormously when comparing the two platforms. A $1B Kalshi week and a $1B Polymarket week are not the same dollars at risk. The Polymarket number is closer to economic reality. The Kalshi number is closer to notional convention.

Market diversity and the user-count split

The breadth gap also favors Polymarket.

As of April 2026, Polymarket hosts over 12,000 active markets spread across ten categories, with cumulative all-time volume of $63.4 billion and a record $10.57B traded in March 2026 alone.

User counts compound the breadth advantage.

User counts remained Polymarket's clearest advantage. The platform drew 678,342 unique users in April, more than eight times Kalshi's implied user base.

That ratio — 8x more users, ~37% of the taker volume — implies the average Polymarket trader is much smaller than the average Kalshi trader. Kalshi's growth has come disproportionately from institutional flow.

Kalshi is also pushing deeper into institutional trading. It recently executed its first custom block trade, and institutional trading volume on the platform has reportedly grown 800% over the past six months.

This is the cleanest framing of the divide. Polymarket is the global retail venue. Kalshi is the U.S. institutional venue with a sports retail layer bolted on via Robinhood. They are not really competing for the same trader.

The Counter-Argument

The strongest case against the "Kalshi has won" narrative comes from Polymarket itself, and from analysts who scrutinize how Kalshi counts.

Kalshi's John Wang argued in March that Kalshi processed $13 billion against Polymarket's $10 billion, but Wang himself argued comparisons are flawed, citing sports-heavy US volume and war-related contracts for Polymarket. He also alleged up to 70% wash trading in some Polymarket markets, but it is still unverified.

Wash trading cuts both ways. If Polymarket has unverified manipulation in its top markets, Kalshi has notional-inflation conventions that arguably overstate its lead. Both critiques deserve weight; neither has been independently confirmed.

The deeper counter-argument is about category mix, not measurement. Kalshi's lead exists primarily because it captured U.S. sports — a category that didn't really exist on Polymarket before 2024. Strip out sports, and the comparison changes radically. Polymarket leads decisively in politics (92.6% market share), geopolitics (essentially 100%), international tournaments (99.3% of World Cup volume), and high-fee crypto markets.

There is also the Polymarket US wildcard.

In July 2025, Polymarket spent $112 million to acquire QCEX, a CFTC-registered exchange and clearinghouse. The CFTC issued an Amended Order of Designation on November 25, 2025, and the U.S. platform launched in beta the following week with sports markets only.

Polymarket US is a distant third at around $5 million in weekly volume with roughly 440 active markets and $650,000 in open interest. The platform is early, and those numbers will grow as the waitlist clears.

If Polymarket US gets the global brand, the $29M/month fee engine of the international platform, and even modest U.S. distribution, the picture changes again within twelve months.

Finally, there is the growth question.

Prediction market monthly trading volumes are cooling. The slowdown was concentrated on Polymarket's global platform, where active traders fell to about 643,000 in April from more than 733,000 in March.

Kalshi is gaining share partly because the overall pie is contracting and Polymarket is contracting faster. A "win" earned on a shrinking base is less impressive than a win earned on a growing one.

What I'm Watching

1. The June FIFA World Cup as a stress test.

How each platform captures World Cup volume will be a defining test in the sports category this summer.

If Polymarket's 99% share of pre-tournament volume holds through the group stage, the "Kalshi owns sports" thesis weakens. If Kalshi's parlay product converts even partial tournament flow, its lead becomes structural.

2. Polymarket US graduating from beta. Watch for a public timeline. The current $5M/week trickle is irrelevant. The question is whether Polymarket can convert any meaningful share of its 678,000 global monthly users into U.S. accounts once geographic restrictions lift — and whether the brand survives the regulatory rebranding.

3. The CFTC's framework determination on swaps vs. futures.

On March 12, 2026, the agency began a formal process to build a framework for prediction markets. It sought public input on whether event contracts should be classified as "swaps" or "futures".

The classification matters enormously for ETF wrappers, institutional participation, and Polymarket US's product scope.

4. Kalshi's institutional trajectory.

Clear Street's participation as the first institutional FCM on Kalshi's exchange (joined May 2026) opens access to institutional trading desks, ETF issuers (via swap capabilities), and block-trading workflows that retail-only infrastructure could not support.

If a second and third FCM follow Clear Street within six months, the institutional moat hardens.

5. The 2028 presidential primary market depth. Politics is Polymarket's last clear moat. If Kalshi's 2028 Democratic Nominee market — which already shows substantial open interest on light flow — starts attracting active trading rather than passive positioning, the politics monopoly breaks. If it doesn't, Polymarket retains its identity as the venue for serious political price discovery regardless of how the sports race plays out.

The answer to "who wins" is finally clear, if you ask the right question. Kalshi wins on headline U.S. volume, sports liquidity, and institutional infrastructure. Polymarket wins on fees, market breadth, political and geopolitical depth, and global users. Both can be true. The platforms have stopped fighting for the same trader — and that, more than any volume number, is the most important development in prediction markets right now.