Polymarket's US-regulated prediction market has reached $111.4 million in open interest, nearly doubling since the 2026 FIFA World Cup concluded in mid-July. The platform now accounts for 54% of total Polymarket volume — a striking milestone for a product that did not exist before December 2025.

Open interest — the total value committed to contracts bought but not yet settled — is generally read as a gauge of positioned capital rather than day-to-day activity, which makes the post-tournament climb a more durable signal than a betting-volume spike. The growth is particularly notable because it runs counter to the broader industry trend. After the World Cup wrapped up on July 19, combined open interest across prediction markets — including competitor Kalshi — fell by roughly 20%. Polymarket US moved in the opposite direction entirely.

A World Cup hangover for the rest of the market

By any measure, the 2026 World Cup — the first held in North America since 1994, jointly hosted by the United States, Canada and Mexico — was a bonanza for prediction markets. Polymarket's soccer category alone generated over $2 billion in volume during the tournament's first ten days, while Kalshi's peak combined open interest during the event hovered around $2 billion.

Once the tournament ended, Kalshi's open interest slid from its near-$2 billion peak to approximately $1.5 billion by late July. Polymarket US bucked this pattern: its open interest kept climbing through August, hitting $111.4 million by August 22.

What sets Polymarket US apart

The original Polymarket operates as a crypto-native platform where users post collateral on-chain. It exited the American market after a 2022 settlement with the Commodity Futures Trading Commission, under which it paid a $1.4 million penalty and blocked US users. Polymarket US, in contrast, runs entirely on fiat USD contracts with full KYC compliance — no crypto collateral and no anonymous wallets.

The platform emerged from Polymarket's acquisition of QCEX in December 2025, a CFTC-registered designated contract market whose license gave it the regulatory framework needed to serve American users legally — the same oversight model under which Kalshi, approved as a regulated exchange in 2021, rode election contracts to mainstream attention during the 2024 US presidential race. The fact that a fully regulated, fiat-only prediction market now commands 54% of total Polymarket volume signals where demand is actually coming from: a user base comfortable with KYC requirements and dollar-denominated positions, rather than crypto traders seeking leveraged exposure to obscure outcomes.

Longer holds, less churn

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

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

What this means for the prediction market landscape

Kalshi still dwarfs Polymarket US in absolute open interest, at roughly $1.5 billion versus $111.4 million. But Kalshi's number is trending down while Polymarket US's is trending up. The next fixed point on the calendar is the November 2026 US midterm elections — the category's first national vote since Polymarket returned to the American market — when many of the longer-dated positions now holding open interest will begin to resolve, offering a clearer read on whether regulated fiat demand sustains outside a tentpole sporting event.