Polymarket's US Exchange Overtakes International

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Last week we published Novig Strikes Home With Their First Billion Dollars, on the sports-only exchange crossing $1 billion in notional three days after its Sydney Sweeney ad.

Check it out if you haven’t already here!

Polymarket's US exchange traded $3.92 billion in notional volume over the first eighteen days of September 2026. Its offshore book traded $2.93 billion over twenty days, so the US side is pushing ahead.

It is the first month the regulated US exchange has been larger.

Measured per day, offshore volume is effectively flat compared to August 2026, and the US exchange is up 76%.

Polymarket US is predominantly a sports venue. Politics, economics and crypto combined equate to 0.6% of everything traded since launch. The other 99.4% is sport.

The exchange traded $3.83 billion across the whole of August 2026. The two busiest days in its history are that first NFL weekend, $346.8 million on the Saturday and $373.6 million on the Sunday.

American football went from $145 million in August 2026 to $673 million in the first eighteen days of September 2026. Combos, where several outcomes are bundled into one ticket, went from $395 million to $1.42 billion.

This comes at a time when the offshore book is seeing regulatory pressure from other directions. South Korean police charged 26 Polymarket users last week, Europe's top regulator has questioned the venue's EU access, and Americans who once reached the offshore book now have a legal domestic route.

On 16th September 2026, a three-judge Ninth Circuit panel held that Kalshi's sports event contracts are Class III gaming under the Indian Gaming Regulatory Act when a customer buys them on tribal land.

Kalshi says its federally regulated event contracts sit outside state gambling rules, and the tribes say that, on their land, a contract on a football game is still sports betting.

Tribal gaming has its own legal system. A 1987 Supreme Court ruling stopped states from regulating gambling on tribal land. Congress responded a year later with the Indian Gaming Regulatory Act.

The Act splits gaming into three classes.

A tribe normally needs a compact negotiated with its state to offer Class III gaming. The state gets a share of gaming revenue, and the tribe gets protection from competing gambling operators.

On that bargain, tribes took $46.2 billion in gross gaming revenue in the 2025 financial year, across 246 tribes in 29 states, per the National Indian Gaming Commission, compared to $78.6 billion for commercial casinos.

In California, on 8th November 2022, Proposition 27 asked voters to legalise statewide mobile sports betting run by commercial operators. More than 50 tribes campaigned against it, and 82% of voters rejected it.

California still has no legal retail or online sports betting. Prediction markets are now one of the few legal ways to put money on a game in the state. The two tribes suing Kalshi are both Californian.

The tribes won that vote by more than four to one, and the product arrived anyway on a federal licence, with no compact, no state process and no revenue share.

Every ruling that treats event contracts as gambling rather than derivatives pushes Kalshi and Robinhood ($HOOD ( ▲ 3.71% )) towards the state-by-state licensing and tax that DraftKings ($DKNG ( ▲ 2.02% )) and Flutter ($FLUT ( ▼ 2.71% )) already pay.

Two days after the Ninth Circuit ruling, the Tunica-Biloxi Tribe of Louisiana launched SaltTrade Derivatives on 18th September 2026, the first event-contract platform owned by a sovereign tribal nation.

The Tunica-Biloxi are one of the same federally recognised tribal nations covered by the Indian Gaming Regulatory Act.

They operate the Paragon Casino Resort in Louisiana, but instead of challenging Kalshi, they are routing SaltTrade's trades through Kalshi's exchange and sharing revenue with the company.

SaltTrade will not register with the CFTC. It is operating under a conditional no-action letter, which means the regulator has said it will not take enforcement action under specified conditions, rather than formally approving or registering the platform.

For Kalshi, the partnership gives chief executive Tarek Mansour a useful answer while 17 tribes ask the CFTC to treat prediction markets as gaming.

"The debate can no longer be reduced to prediction markets on one side and tribes on the other," Mansour said.

Tunica-Biloxi, with roughly 1,600 members and one casino, is much smaller than the California tribal gaming interests trying to block Kalshi.

The Wall Street Journal reported on 20th September 2026 that Polymarket faced an attempted theft of at least $10 million in stolen debit-card deposits, and despite this, the chief executive Shayne Coplan pushed the company to keep growing while its compliance team raised alarms.

Fraudsters began targeting the US platform in February 2026. Checkout.com rejected more than 80% of the deposits it handled as fraudulent, compared to an industry norm of roughly 1%.

A second attack followed in late July 2026, compromising nearly 500 accounts through a flaw in account registration, using stolen personal data including Social Security numbers rather than passwords.

This comes against a backdrop where Polymarket has spent more than 4 years building its regulated American business.

The priority on growth comes as no surprise given the competition Polymarket is facing from all sides.

On 21st September 2026, CoinDesk reported that Kalshi are facing allegations of fake trading volume on their ether perpetual futures contract.

Beni, a quantitative analyst and co-founder of Stealth Neolab, posted on X that the contract logged $539 million in 24-hour trading volume compared to $3.1 million of open interest, a ratio of 174 to 1. He said identical $5,500 trades made up as much as 58% of the contract's volume across four separate days.

He also pointed to a rebate schedule Kalshi filed with the CFTC, under which a 0.3 basis point maker rebate offsets a 0.3 basis point taker fee, leaving some self-clearing members paying a net fee of zero.

IcoBeast, who oversees product development at Kalshi, replied on X that any firm meeting the regulatory requirements can become a self-clearing member, and that Kalshi do not offer rebates on their crypto event contracts. He said the Artemis chart cited in the original post measures prediction-market share, and that Kalshi have to file every incentive programme publicly with the CFTC.

He added that Kalshi's headline volume counts the maximum payout on every contract, the same convention Polymarket use, so a trader buying 100,000 contracts at 30 cents spends $30,000 and records $100,000 of volume.

"I'm the first to admit that it's early days for perps for us given we're building a new product in untrodden territory," he wrote.

Kalshi had not responded to CoinDesk's request for further comment at the time of publication.

Predicted is written by Pet Berisha and Omar El Safy. Every edition, our quarterly State of Prediction Markets reports and more at predicted.co.

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