Kalshi and Polymarket recorded a combined $45.33 billion in trading volume in August, ending a year-long run of monthly growth as activity cooled after a World Cup-driven summer surge. The total was down 14.5% from July and marked the first month-over-month decline since August 2025, according to the platforms’ reported figures.
Kalshi accounted for most of the market, handling $37.17 billion in August. That represented a 7.3% decrease from its $40.1 billion July total. Polymarket and Polymarket US combined for $8.16 billion, falling 36.7% from $12.89 billion in the previous month.
The decline leaves both operators well above their spring activity levels. Combined volume reached $25.66 billion in May, meaning August trading remained roughly 77% higher despite the monthly retreat. The figures point to a market that has retained much of its expanded user activity after a major global sporting event, even as the event-specific contracts that lifted summer volumes began to settle.
World Cup contracts lifted summer volumes
The FIFA World Cup, which ran from June 11 through July 19, helped fuel a sharp increase in event-contract trading across the platforms. Major sports tournaments generate a dense schedule of short-duration markets, allowing users to trade on individual matches, player outcomes, team progress and tournament results.
That structure can produce large turnover without necessarily creating lasting volume at the same level once the event ends. August’s slowdown was steeper at Polymarket and Polymarket US than at Kalshi, suggesting that the two operators did not retain the same mix of post-tournament activity.
Kalshi’s $37.17 billion August figure gave it more than four-fifths of the combined volume reported by the three venues. Its smaller percentage decline also widened the gap with Polymarket, whose August total was less than a quarter of Kalshi’s.
The summer figures illustrate how prediction-market volumes can become concentrated around a small number of high-profile events. Sports contracts have become one of the industry’s most commercially valuable products, but they have also placed federally regulated event-contract platforms in direct conflict with state gambling authorities.
State challenges intensify around sports contracts
More than a dozen U.S. states have brought enforcement actions, issued warnings, or launched lawsuits involving sports-related contracts offered by Kalshi and Polymarket, according to the supplied information. State regulators generally argue that contracts tied to sports outcomes resemble betting products subject to state licensing and consumer-protection rules.
Kalshi has maintained that its contracts fall under federal commodities regulation, a position that has produced an expanding patchwork of court disputes. The legal uncertainty has become a practical business issue for platforms seeking to offer nationwide products while states argue that local gambling laws remain enforceable.
Connecticut filed suit last week seeking to block Kalshi from offering sports contracts in the state. The case extends a dispute that has been building for months and adds to the pressure on Kalshi’s model of making event contracts available across jurisdictions.
A U.S. Court of Appeals for the Ninth Circuit ruled on Aug. 28 that Nevada can enforce its gambling rules against the platforms, according to the supplied account of the decision. The ruling supports the Nevada Gaming Control Board’s position in litigation argued by attorney Kannon Shanmugam Saharsky on the regulator’s behalf.
That outcome sits uneasily beside an earlier New Jersey order that favored a different approach to the state-versus-federal jurisdiction question. Divergent rulings across federal courts could eventually force the issue toward the U.S. Supreme Court, though no Supreme Court review has been granted.
The immediate effect is likely to be operational complexity rather than a single nationwide outcome. Platforms may face different restrictions, legal costs and product availability rules depending on the state in which users are located. Sports contracts, rather than political markets, are currently at the center of the most active state-level challenges described in the supplied material.
Kalshi expands sports ties as legal cases continue
Kalshi has continued to pursue major sports partnerships while defending its legal position. The company signed an agreement with the U.S. Tennis Association to become the exclusive prediction-market partner of the U.S. Open, whose main draw began Aug. 30.
The partnership gives Kalshi a prominent role around one of the largest annual events on the U.S. tennis calendar. It also arrives as regulators scrutinize whether event contracts connected to sports should be treated differently from conventional financial products.
The U.S. Open arrangement places Kalshi’s brand before a mainstream sports audience at a sensitive moment for the sector. A favorable legal outcome would allow federally regulated platforms to keep pursuing sports relationships and contracts on a national scale. State victories could limit that strategy market by market.
Kalshi also took a separate enforcement step involving former U.S. Representative George Santos. The platform permanently banned Santos after he placed a contract on whether he would attend the State of the Union, according to the supplied information.
Santos was ordered to pay more than $71,000 in fines connected with the State of the Union trade. Kalshi described the action as its first permanent user ban, showing that platform compliance issues can extend beyond the state-law disputes surrounding sports markets.
Election activity could reshape the volume mix
The August decline came before the U.S. election season reaches its busiest period. Political contracts have historically drawn large volumes when major campaign developments, debates, polling shifts and election outcomes create rapidly changing probabilities.
That could give Kalshi and Polymarket a different source of activity in the coming months, though political-market demand would not resolve the legal questions surrounding sports contracts. The platforms’ challenge is to sustain growth across several categories while regulators, courts and state gaming agencies continue to define the boundaries of their businesses.
August’s figures show that the World Cup boost was difficult to maintain after the tournament ended. Yet with $45.33 billion in combined monthly trading, the two platforms remain far larger than they were before the summer surge, while their legal fight over sports contracts is becoming more geographically fragmented.
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