Kalshi captured 70.8% of combined US prediction-market notional volume in the week ending May 24, 2026, with sports contracts driving $1.6 billion of its $3.99 billion weekly total — and the June 3 NBA Finals tip-off will decide whether that share number is a regime change or a one-cycle anomaly.
Kalshi took $3.99 billion of weekly notional volume in the seven days ending May 24, 2026, against Polymarket’s $1.65 billion, lifting the CFTC-regulated exchange’s share of combined prediction-market volume to 70.8% — its highest reading on record (DeFi Rate weekly tracker, May 25, 2026). Of that $3.99 billion, $1.599 billion came from sports contracts alone, a category Kalshi did not list before late 2024. This Deep Dive walks through the volume split, the structural reasons Kalshi has out-scaled Polymarket on US sports, the NBA Finals contracts that will test the model in June, and the four observable signals that would prove the “Kalshi-dominant” thesis wrong.
Key Facts:
• Kalshi weekly notional volume: $3.99 billion, +3% week-over-week — DeFi Rate, May 25, 2026
• Polymarket weekly notional volume: $1.65 billion, -15% week-over-week — DeFi Rate, May 25, 2026
• Kalshi sports-category volume: $1.599 billion (40.1% of Kalshi total) — DeFi Rate, May 25, 2026
• Polymarket sports-category volume: $675.5 million (40.9% of Polymarket total) — DeFi Rate, May 25, 2026
• Oklahoma City Thunder NBA Finals contract: 64.5¢ implied probability, $13.2 million+ open interest — Bettors Insider, May 23, 2026
• Kalshi 2025 revenue mix: 89% from sports contracts, on more than 90% of platform activity — Yahoo Sports, May 2026
• Kalshi valuation: $22 billion post-TCV-led funding round, May 2026 — NPR / Sportico
Methodology
The volume figures come from DeFi Rate’s weekly prediction-market tracker, which aggregates publicly available Kalshi and Polymarket disclosures for the rolling seven days through Saturday’s close (May 18-24, 2026 in this window). Category splits — sports, crypto, politics, culture/entertainment, economics — are taken from the same tracker, which classifies contracts using each platform’s own market labels. NBA Finals contract pricing and open interest figures are from the Bettors Insider weekly prediction-market roundup snapshot dated May 23, 2026, cross-checked against Polymarket’s public API and Kalshi’s market pages on the same day. We use Kalshi’s “sports incl. exotics” category as comparable to Polymarket’s “sports” classification; the two platforms use different exotic-contract definitions, so the totals are directionally comparable rather than exactly like-for-like.
Limits: this is a single-week snapshot. Weekly volumes on prediction markets are highly event-driven (May 2026 captured Stanley Cup Conference Finals, NBA Conference Finals, and the buildup to the FIFA World Cup 2026 outright market), so any single-week share figure overstates persistence. A multi-week regression would be more robust but is outside the scope of this snapshot. We also exclude offshore Polymarket international volume that is not captured by the DeFi Rate tracker, which biases the Kalshi share number upward by an unknown but probably single-digit-percentage amount.
The data: where the volume sits
The headline split is straightforward — Kalshi sports volume now exceeds Polymarket sports volume by approximately 2.37× in dollar terms — but the category mix tells a sharper story. Sports is the largest single category for both exchanges, but Kalshi’s sports volume is 19.4% larger than Polymarket’s entire weekly notional across all categories combined.
| Category | Kalshi (USD, week ending May 24, 2026) | Polymarket (USD, same window) | Kalshi share of category |
|---|---|---|---|
| Sports (incl. exotics) | $1,599,000,000 | $675,500,000 | 70.3% |
| Crypto | $370,000,000 | $399,000,000 | 48.1% |
| Politics | $36,000,000 | $211,700,000 | 14.5% |
| Culture / Entertainment | $5,700,000 | $13,200,000 | 30.2% |
| Economics | $8,400,000 | $17,900,000 | 31.9% |
| All-category total | $3,990,000,000 | $1,650,000,000 | 70.8% |
Sources: DeFi Rate weekly prediction-market tracker, May 25, 2026, drawing on Kalshi and Polymarket public market data for trades settled May 18-24, 2026.
The structural reading: Kalshi’s lead is concentrated in sports and crypto, while Polymarket retains the politics franchise that built its 2024 brand recognition. That matters because sports volume has historically dwarfed politics volume on a recurring basis — politics is a quadrennial spike, sports is a 12-month calendar — and the relative weighting is now starting to dominate the platform-share numbers in ways the 2024 presidential cycle obscured.
The most-watched single contract heading into June is the NBA Finals champion market. As of May 23, 2026, Oklahoma City Thunder traded at 64.5 cents implied probability with more than $13.2 million in open interest, ahead of San Antonio Spurs at 15 cents and Cleveland Cavaliers at 2 cents (Bettors Insider, May 2026). The New York Knicks — fresh off a 4-0 Eastern Conference Finals sweep — were not listed as a separate Finals contract on Polymarket as of that snapshot, instead trading on game-level matchup pricing until the East side closed.
“The long-term vision is to financialize everything and create a tradeable asset out of any difference in opinion.”
— Tarek Mansour, CEO, Kalshi
(Bloomberg Opinion)
The mechanism: why Kalshi is winning sports volume
Three structural factors explain why Kalshi has out-scaled Polymarket specifically on US sports, even as Polymarket retains primacy on international politics. First, Kalshi’s CFTC-regulated wrapper means US residents can deposit from any bank account without using a crypto on-ramp — a friction point that has plagued Polymarket’s US-eligible product since its October 2024 re-launch. Second, Kalshi’s order-book structure pulls in arbitrageurs from CME/Eurex desks who are uncomfortable with Polymarket’s USDC-on-Polygon settlement and the related custody assumptions. Third, partnership and self-certification velocity: Kalshi self-certified parlay-style sports contracts with the CFTC in early 2026, an option Polymarket’s US arm has only partially mirrored.
The peer-to-peer-exchange framing also helps Kalshi against the regulated sportsbooks themselves. BetMGM cut its 2026 revenue outlook explicitly citing prediction-market competition, and Mansour has publicly contrasted Kalshi’s exchange model with traditional books that are “essentially a product that is designed for customers to lose.” Whether or not that argument is technically accurate — sportsbook commissions, prediction-market fees, and house edges all carry economic-equivalent friction — the marketing framing is moving handle, particularly among professional bettors squeezed by US sportsbook limits.
Steelmanning the contrarian view: the May 2026 split is heavily flattered by the NBA postseason, the Stanley Cup Conference Finals, and the buildup to the FIFA World Cup 2026 outright. Once those three event clusters resolve, July-August 2026 prediction-market volumes typically compress 30-45% versus peak-postseason weeks. A snapshot that captures peak event density risks treating a cycle high as a steady state.
Where this thesis breaks
The “Kalshi-dominant” reading rests on three assumptions that could fail in the coming six weeks:
- Polymarket sports liquidity recovers post-Finals. If Polymarket’s US arm closes its CFTC self-certification gap on parlay-style markets and rolls out FIFA World Cup contracts at sub-$0.02 spreads, the sports-volume gap could close by 15-25 percentage points within four weeks. Watch for Polymarket-US product announcements in the first half of June.
- Sportsbook handle holds. The thesis assumes prediction-market growth is partly substitution for sportsbook handle. If American Gaming Association (AGA) monthly handle reports for May and June show flat or growing sportsbook handle alongside the prediction-market surge, the substitution narrative weakens — and the share number becomes a co-existence story rather than a winner-takes-most story.
- CFTC re-classification. A CFTC or state-court ruling that re-classifies sports prediction contracts as illegal swaps under Commodity Exchange Act §1a(47) would compress Kalshi’s sports category immediately. Texas, New Jersey, and Massachusetts have all filed amicus interest in pending cases.
- NBA / NHL official partnership with sportsbooks gains exclusivity. If the NBA or NHL grants a partnership to either prediction-market platform on an exclusive basis, the loser’s volume could collapse by 20-40% on those league contracts.
None of these are tail events. The June 8 Polymarket product roadmap update, the June 10 AGA monthly handle release, and the June 17 SCOTUS conference where prediction-market related cases sit on the docket are the three near-term dates that could each tip the picture.
“The vision that I know that my team and I want to build has not come to life fully yet.”
— Shayne Coplan, CEO and Founder, Polymarket, in May 2026 Harvard remarks
(Coplan public profile / Bloomberg coverage)
Prior cycle: what 2024 election volume tells us
The relevant analogue is not a sports cycle — it’s the 2024 US presidential election cycle, when Polymarket built its brand and Kalshi’s pre-launch caution kept it on the sidelines. Polymarket peaked at $3.7 billion in November 2024 election-week volume, a number it has never exceeded in any single sports week since. Kalshi’s first $1 billion-plus sports week, by contrast, came less than 12 months after it began listing sports contracts. The implication: prediction-market category economics shift toward whichever platform has the regulatory wrapper that matches user demand at the moment.
Polymarket’s political category is durable; its sports category is structurally disadvantaged in the US until and unless its CFTC posture catches up. Polymarket’s broader move off crypto rails — toward more compliant fiat on-ramps — is partly an acknowledgement that the next category of revenue is sports, not politics, and sports volume responds badly to USDC-only deposits.
What this means for the NBA Finals stress test
The NBA Finals begin June 3, 2026, and run through approximately June 19, 2026 in a 4-3 series scenario. The Finals are the single largest prediction-market event in a typical sports calendar: in 2025, the seven-game Boston-Indiana Finals produced over $480 million in prediction-market sports volume across both platforms (DeFi Rate historical aggregation). The 2026 edition will produce more, given the larger user base and the OKC vs. Knicks / Spurs match-up dynamics.
Three sub-markets to watch by series-end:
- Finals MVP contract liquidity — historically lags the champion contract by 6-8× in volume; if it closes that gap, retail prop interest is shifting onto prediction markets in a way that would worry sportsbook risk managers.
- Game-level matchup contract pricing vs. Vegas spreads — Kalshi pricing has tracked Pinnacle within 1-2 cents implied probability in 2026 game-level NBA contracts. Wider divergence during the Finals would signal liquidity stress.
- Polymarket vs. Kalshi same-market spread — currently typically 0.5-1.5 cents wide. If that widens past 3 cents on Finals contracts, professional arbitrageurs are stepping back, and the platform-share dynamics will swing again.
The companion structural angle from prior coverage is that stablecoin flows have already decoupled crypto-casino GGR from Bitcoin’s price, and prediction markets are now showing a similar decoupling from crypto liquidity cycles. The Finals stress test will help quantify it. Q1 2026 gambling-sector earnings already flagged the substitution dynamic; June and July reporting will sharpen the picture.
What to watch next
Three specific data points in the next six weeks will tell whether the May share number was a peak or a starting point:
(1) The DeFi Rate weekly tracker reading for June 14, 2026 — the first full week of NBA Finals trading — will show whether Kalshi’s sports share has crossed 75% or compressed back toward 65%. (2) The AGA Commercial Gaming Revenue tracker for May 2026, due mid-June, will provide the sportsbook-handle counter-reading. (3) Any CFTC enforcement filing in the week of June 16 connected to sports prediction-market self-certification — a single adverse notice would compress Kalshi’s sports volume by 10-20% within days of release.
TL;DR
Kalshi took 70.8% of combined US prediction-market volume in the week ending May 24, 2026, driven by $1.599 billion of sports notional against Polymarket’s $675.5 million (DeFi Rate, May 25, 2026). The CFTC-regulated wrapper, order-book structure, and parlay self-certification have given Kalshi a structural lead specifically in US sports. The June 3 NBA Finals tip-off is the first major stress test of whether that lead persists. The thesis breaks if Polymarket closes its parlay-product gap, sportsbook handle holds steady, or the CFTC re-classifies sports contracts.
FAQ
What’s the difference between Kalshi and Polymarket on sports?
Kalshi is a CFTC-regulated derivatives exchange where users deposit US dollars from a bank and trade event contracts in an order book. Polymarket’s US arm is also CFTC-regulated but settles in USDC stablecoin on Polygon, requiring a crypto on-ramp. The fee model, settlement currency, and onboarding friction are the operational differences that explain most of the 2026 sports-volume gap.
Are prediction markets the same as sports betting?
Legally, no — Kalshi and Polymarket contracts are derivatives under the Commodity Exchange Act, not licensed sportsbook bets. Economically, the user experience is similar (stake on an outcome, win a payoff). The CFTC and several state attorneys general are actively litigating whether the legal distinction holds for sports-specific contracts.
How big is the prediction-market category compared to US sportsbooks?
The American Gaming Association reported approximately $11.3 billion in commercial sportsbook revenue in 2024. Combined prediction-market notional volume reached $5.64 billion in a single week of May 2026 — roughly a 50-week run-rate of $290 billion. Notional volume and revenue are different metrics, but the gap between the two industries on volume terms is closing fast.
Why does the NBA Finals matter for this thesis specifically?
The Finals are the single largest sports prediction-market event of the calendar year. They will produce the cleanest single-event test of whether Kalshi’s structural advantages compound, or whether Polymarket recovers share once the US arm rolls out competitive sports product. A 65-70% Kalshi share through the Finals would suggest the May reading was directional; a sub-60% share would suggest it was a cycle blip.
Where can US bettors access these markets legally?
Kalshi is available to US residents in all 50 states under its CFTC designation, subject to state-level enforcement letters that have been filed in several states. Polymarket’s US-regulated arm is restricted by jurisdiction depending on which contracts a user attempts to access. Always check current state guidance.
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