World Cup boom sends Polymarket volume up 300% while Kalshi sets open interest records

Polymarket's "Soccer" category saw over $2 billion in volume during the first ten days of the World Cup, a 300% increase over the prior ten-day period. At the same time, daily average volume for the category grew from $53 million in the month before kickoff to about $220 million during the tournament so far.

The mechanical driver behind this growth is straightforward: the world’s most popular, most-anticipated, quadrennial tournament, having multiple daily sports matches for an entire month, where each match is a short-dated event market that resolves within 90 minutes.

Polymarket isn't the only prediction market that has enjoyed the fruits of the World Cup, as Kalshi's aggregated open interest (OI) reached a record $1.16 billion last Thursday, the first time the platform’s OI surpassed the billion-dollar mark. This marks a 350% year-to-date increase for Kalshi's OI.

Interestingly, Polymarket's OI levels have remained relatively flat even during the World Cup, while the OI in Polymarket's U.S. arm, which is also CFTC-regulated like Kalshi, has only increased modestly, failing to even reach its April 2026 highs.

The more interesting story to highlight, therefore, lies in the Polymarket-Kalshi OI divergence.

Kalshi's OI has been compounding faster than its trading volume, indicating a user base with longer hold periods on betting positions and the emergence of larger directional positioning rather than scalping. Kalshi's CFTC-regulated rails and direct USD on-ramp give it a structural edge for U.S. institutional and high-net-worth dollar investors who prefer not to custody offshore, and the World Cup is the first event in Kalshi's history large enough to surface that demand at scale.

Kalshi, being a regulated U.S. sportsbook with a settlement engine, also means it is competing directly with the likes of DraftKings and FanDuel, rather than just Polymarket U.S.

This is an excerpt from The Block's Data & Insights newsletter. Dig into the numbers making up the industry's most thought-provoking trends.