Polymarket US open interest nearly doubles to $111M after World Cup winds down

Polymarket’s US-regulated prediction market has hit $111.4 million in open interest, nearly doubling since the 2026 FIFA World Cup wrapped up in mid-July. The platform now accounts for 54% of total Polymarket volume, a striking milestone for a product that didn’t exist until December 2025.

The growth is especially notable because it runs counter to the broader trend. After the World Cup concluded on July 19, combined open interest across prediction markets, including competitor Kalshi, dropped by roughly 20%. Polymarket US, meanwhile, went the other direction entirely.

The World Cup hangover hit everyone else harder

The 2026 World Cup was, by any measure, a bonanza for prediction markets. Polymarket’s soccer category alone generated over $2 billion in volume during the tournament’s first ten days. Kalshi’s peak combined open interest during the event hovered around $2 billion.

Kalshi’s open interest slid from its peak near $2 billion down to approximately $1.5 billion by late July. Polymarket US bucked this pattern. Its open interest continued climbing through August, reaching $111.4 million by August 22.

What’s different about Polymarket US

The original Polymarket operates as a crypto-native platform where users post collateral on-chain. Polymarket US, by contrast, runs entirely on fiat USD contracts with full KYC compliance. No crypto collateral, no anonymous wallets. The platform was born out of Polymarket’s acquisition of QCEX in December 2025, giving it the regulatory framework to legally serve American users.

The fact that a fully regulated, fiat-only prediction market is now commanding 54% of total Polymarket volume says something about where demand is actually coming from. It’s not degens looking for leveraged exposure to obscure outcomes. It’s a user base comfortable with KYC requirements and dollar-denominated positions.

Longer holds, less churn

During the World Cup, match-by-match betting created a churn machine where positions opened and closed within hours. The contracts were short-dated by nature, tied to specific games with specific outcomes on specific days.

With the tournament over, the contracts that remain tend to be longer-dated. Higher open interest with lower volume is the signature of a market shifting from speculation to positioning.

What this means for the prediction market landscape

Kalshi still dwarfs Polymarket US in absolute open interest, with roughly $1.5 billion versus $111.4 million. But Kalshi’s number is trending down while Polymarket US’s is trending up.

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