The Build Outpaced The Volume.
$45.3 billion in August, the category's first monthly decline in a year. Kalshi's volume held far better than Polymarket's, lifting its share to 82%. Underneath the slower month, the building continued. Cantor opened Kalshi to 3,000 clients, five MLB teams and the US Open signed on, and Kalshi filed a $1.5 billion offering.
August broke the streak. Concentration didn't.
Combined volume across Kalshi, Polymarket and Polymarket US fell 14.5% in August to $45.33 billion, according to The Block's dashboard, the category's first month-over-month decline in a year. The pullback followed the World Cup-driven surge that carried June and July, and it still left August comfortably above May's $25.66 billion. A cooling month, not a reversal.
The split matters more than the total. Kalshi recorded $37.17 billion, down 7.3% from a revised July figure of $40.1 billion. Polymarket and its US platform recorded a combined $8.16 billion, down 36.7%. Kalshi's decline was roughly a fifth the size of Polymarket's, so even as the category shrank, Kalshi's own share of it grew, from 75.7% in July to 82.0% in August.
The same caveat from last month still applies. These are taker notional volume figures, not revenue and not deposits, and a single contract can trade several times before it settles. What August demonstrates is that the World Cup was a genuine spike rather than a new baseline, and that the spike is unwinding toward Kalshi rather than away from it.
Kalshi's own decline was a fifth the size of Polymarket's. The category got smaller and more concentrated in the same month.
The retention test from last month has an answer
Last edition flagged the second half of the year as a retention question: which venues convert World Cup traders into recurring users, and which captured a spike and lost it. August is the first real data point, and it lands as a genuine but bounded cooldown rather than a collapse.
Every platform fell. Kalshi fell least. That single fact, more than the headline decline, is what the month actually demonstrates about where demand is settling once the tournament stops carrying it.
August's total still sits about 77% above May's pre-surge baseline of $25.66 billion. The category kept most of its summer gains, and Kalshi kept the larger share of them.
For operators, the shape of the drop matters more than its size. It fell unevenly, in the same direction every other signal this month points.
Volume cooled. Private-market pricing didn't.
Kalshi's SEC filing shows it has sold $1.12 billion of a $1.5 billion private offering opened in April, on top of the $1 billion Series F it closed in May at a $22 billion valuation. It is now reportedly in talks with Sequoia and Wellington for a further $750 million round that would value it near $40 billion, nearly double its May mark. Annualized revenue is reported to have reached about $4 billion in July, a figure sourced to The Information rather than confirmed by Kalshi.
Polymarket's arc ran the same way from a different start. Intercontinental Exchange, parent of the New York Stock Exchange, committed up to $2 billion in October 2025 at roughly an $8 billion valuation and completed a $600 million tranche in March. In August, ICE chief Jeff Sprecher said it is weighing more as Polymarket seeks a round above $20 billion, more than double where ICE first came in.
Not all of it is closed. The $22 billion round and the filed offering are done. The $40 billion valuation and ICE's next tranche are still in talks. The direction is the same regardless of which reported figures hold.
Whether the license is CFTC-native or the audience is offshore-crypto-native, external capital is now pricing this category several multiples higher than it did in the spring.
Institutional infrastructure kept compounding through the slower month
None of the deals below waited on August's volume number. Each one adds event contracts to distribution or infrastructure that already existed, rather than building a new exchange to house them. That is the pattern worth watching for any platform operator.
Kalshi
Nasdaq-grade surveillanceA multi-year deal to run Nasdaq's market surveillance technology across its markets, formatting trade data for CFTC submission, the same stack major exchanges use.
Kalshi
Wall Street-speed data feedIntegrating DoubleZero's low-latency transport into its order book, a dedicated fiber connection instead of the public internet, the model NYSE, Nasdaq and CME use.
Cantor Fitzgerald
3,000 clients · one brokerLaunched institutional block trading in Kalshi event contracts on August 19, acting as introducing broker to roughly 3,000 clients, with Susquehanna pricing.
Gemini & Apex
Exclusivity LOIA letter of intent making Gemini Titan the exclusive CFTC-regulated venue for crypto event contracts across Apex's futures commission merchant network.
River Markets
$8.5M seed · Haun VenturesThe Y Combinator-backed startup raised an $8.5 million seed round to build a unified execution layer across Kalshi, Polymarket and other venues.
Plus500 & Genius Sports
Incumbents reallocatingPlus500's event contracts line is tracking toward $140M in annualized revenue. Genius Sports disclosed new official data deals with both Kalshi and Polymarket.
What these share is distribution. Cantor added Kalshi to an existing client book, Apex made Gemini's venue available across its FCM network, and River Markets is funding the layer that ties venues together. None of them built an exchange to do it, and that is the route most operators take to add the category.
Kalshi turned distribution into a broadcast strategy
Kalshi signed multi-year marketing partnerships with five MLB teams this month, the Braves, Red Sox, Dodgers, Padres and Giants, covering stadium signage, LED boards, radio and fan activations. Baseball trading volume on the platform is already up 36 times year over year. Separately, Kalshi and The Weather Company will integrate Kalshi's weather probabilities into the Weather Channel app and weather.com, while The Weather Company supplies enterprise-grade data to help settle Kalshi's weather contracts. Weather-related volume is up 500% year over year, and the two companies project the category could reach $1.1 billion in annualized volume.
Kalshi also became the exclusive prediction market partner of the US Open through a deal with the US Tennis Association, timed to the tournament's main draw, which began August 30. Between stadium signage, a consumer weather brand and a Grand Slam, signage and broadcast placement are becoming as central to customer acquisition as product and pricing, worth tracking for any operator without an equivalent sports or media relationship to lean on.
Regulatory friction rose alongside everything else
More than a dozen US states have now moved against Kalshi or Polymarket. Connecticut sued Kalshi on August 27 over its sports contracts, joining a fight already underway in several other states.
The bigger decision came on August 28, when the 9th Circuit ruled that sports event contracts are bets rather than federally regulated swaps, letting Nevada apply its gambling laws to Kalshi, Crypto.com and Robinhood. That contradicts an April ruling from the 3rd Circuit, which put the same contracts under CFTC authority, and the split makes Supreme Court review likely. Robinhood says it will appeal. Until the court resolves it, who regulates US sports contracts stays open.
For an operator, the fight is narrow. It turns on US sports contracts and on whether states or the CFTC hold authority. Whether prediction markets are permitted is not in question. Adding the category through regulated infrastructure keeps that risk with the infrastructure rather than the operator's own platform.
Kalshi also issued its first permanent trading ban, against former US Representative George Santos, for trading a contract on whether he would attend the State of the Union, and fined him $71,356 of its own. That came on top of a separate $35,000 settlement Santos had already reached with the CFTC in July, alongside a three-year federal trading ban.
None of this reverses the capital or infrastructure story above; it runs alongside it. The same month the category signed onto MLB stadiums and the US Open, it drew a fresh state lawsuit, a landmark appellate ruling and its first permanent ban.
Scrutiny at this scale follows the category's size. States, exchanges and media companies are all staking out positions because there is finally something large enough to stake them on.
What this means if you run a trading platform
July answered whether prediction markets monetize and where volume settles once users have a real choice. August answers the harder question left open at the end of last edition: what happens once the World Cup leaves the schedule.
On the headline number, the answer is a real but bounded cooldown. Combined volume fell 14.5%, the first monthly decline in a year, and it fell hardest at Polymarket, down 36.7%, the venue most exposed to retail and international flow. Kalshi, the more regulated and institutional side, gave back only 7.3%. The honest reading is that the World Cup spike unwound faster where the audience was more casual.
Upstream of volume, none of that cooled. Kalshi is pricing a private round near double its spring valuation, ICE is weighing a further stake in Polymarket above where its own investment landed a year ago, and a 3,000-client broker relationship, a Nasdaq surveillance deal and a new execution-layer funding round all arrived in the same month the volume fell.
For an operator weighing entry, the retail pullback is less alarming than it looks. Both platforms sit well above their pre-tournament baselines, and a durable prediction markets line does not rise or fall with a single tournament. The regulatory pressure, a dozen states and a split in the federal courts, is the cost of a market now large enough to be worth contesting. The capital, infrastructure and distribution that arrived this month compound on their own timeline, and that timeline is the one an operator is joining.
Key takeaways
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August volume, share and movement data from The Block's volume dashboard and its August 2026 recap, published 2 September 2026. July's Kalshi figure was revised from $37.7 billion to $40.1 billion as trading data completed.
Kalshi's private offering and Form D details from The Block, 26 August 2026. Reported $750 million round and $40 billion valuation from CoinDesk and The Information, 13 August 2026, not confirmed as closed as of publication.
ICE's Polymarket investment history from ICE's own investor disclosures and BigGo Finance reporting, August 2026. The further investment under consideration is reported, not confirmed.
Kalshi/Nasdaq surveillance and Kalshi/DoubleZero data feed details from CoinDesk, 12 August 2026. Cantor Fitzgerald's institutional access to Kalshi from Cantor's own announcement and CoinDesk, 19 August 2026. Gemini/Apex Fintech letter of intent from GlobeNewswire, 24 August 2026.
River Markets seed round from Fortune, 11 August 2026. Plus500 revenue figures from its first-half 2026 results via Pulse2. Genius Sports data agreements from its earnings call via SCCG Management, 8 August 2026.
Kalshi's MLB and Weather Company partnerships from Kalshi's own announcements, August 2026. US Open partnership and Connecticut lawsuit from The Block, 27 August and 2 September 2026. Kalshi's permanent ban and $71,356 penalty against George Santos from Kalshi's disciplinary notice, 31 August 2026; his separate $35,000 CFTC settlement and three-year federal trading ban from July 2026.
The 9th Circuit Court of Appeals ruling permitting Nevada to regulate sports event contracts, and the conflicting 3rd Circuit decision from April 2026, reported by CNBC and The Nevada Independent, 28 August 2026.
All volume figures measure taker notional volume rather than platform revenue or customer deposits.