Spreads between Kalshi and Polymarket widened sharply over the past seven days, with multiple contracts trading more than four points apart on identical outcomes. The reason is familiar: regulated US volume is piling into Kalshi ahead of the Fed meeting and election cycle, while Polymarket’s crypto-native base is pricing a different set of risks. For traders willing to post collateral on both venues, this is one of the better arbitrage windows we have seen since the summer. Below are the five opportunities worth watching, the mechanics behind each, and the frictions that will decide whether the edge is real after fees.
1. Fed November Rate Decision: 6-Point Gap
The widest spread on the board. Kalshi’s “Fed cuts 25bps in November” contract is pricing a 72% probability as of Friday morning, while Polymarket’s equivalent market sits at 66%. That is a six-point gap on an event with hard, binary resolution in under three weeks.
The theoretical trade: buy YES on Polymarket at $0.66, sell YES on Kalshi at $0.72. Net credit of roughly $0.06 per contract pair, locked regardless of outcome. The catch is that Kalshi’s fee schedule claws back about $0.01 per dollar on short positions, and Polymarket’s gas and withdrawal fees eat another $0.005 to $0.015 depending on how you clear USDC. Net edge after costs: 3 to 4 cents per $1 of exposure, which is still strong for a three-week hold.
2. Bitcoin Above $120K by End of Year: 4-Point Spread
Polymarket’s “BTC above $120K on Dec 31” contract is trading at 41%, roughly in line with options-implied probabilities on Deribit. Kalshi’s parallel contract, however, is only at 37%. That four-point gap persists despite the two platforms resolving on the same CF Benchmarks reference price, so there is no definitional edge to explain it.
The driver appears to be flow: Polymarket’s crypto-heavy user base is more willing to pay up for upside bitcoin exposure, while Kalshi’s traditional-finance crowd is still digesting last week’s drawdown. If you believe the true probability sits in the middle, you can leg the trade: short Polymarket at 41, long Kalshi at 37. The challenge is capital efficiency: this is a 10-week hold, so your 4-cent edge annualizes to roughly 20% returns, before accounting for USDC yield you forgo on the Polymarket collateral.
3. GOP Retains Senate Control: 3-Point Spread
With 11 days until the midterm, Senate control markets are getting extremely tight. Kalshi’s “Republicans control Senate after 2026 election” market is at 58%; Polymarket’s version is at 55%. A three-point gap on a market with this much liquidity is unusual and almost certainly reflects Kalshi’s higher share of politically-aligned retail flow concentrated on the Republican side.
Pure arbitrage here is harder because position limits on Kalshi election contracts are tighter than on most other markets, and both venues have seen volume spikes that occasionally produce 1 to 2 point gaps that close within hours. Patient traders can still work the edges, but this is not a set-and-forget trade.
4. Ethereum ETF Approval by Q1 2027: Widening Gap
A quieter but persistent spread. Polymarket’s “ETH staking ETF approved by SEC before April 1, 2027” is at 29%, while Kalshi’s recently-launched equivalent is at 24%. The five-point gap has held for most of the week and may be structural: Kalshi’s regulatory relationship with the CFTC makes some traders assume Kalshi has better insight into SEC timelines, pulling probability down. In practice, the two platforms have no informational edge over each other on this kind of question.
This is a longer-duration trade (roughly five months to resolution), so capital cost matters. At current rates, a 4-cent net edge after fees still clears the bar if you were going to park collateral anyway.
5. “Government Shutdown Resolved by November 15” Mini-Arb
The highest-conviction trade of the five. Kalshi is pricing a resolution by November 15 at 81%; Polymarket at 77%. Four-point spread, three-week hold, and both markets resolve on the same objective trigger (a signed continuing resolution). After fees this is roughly 2.5 cents per dollar of edge, lower than the Fed trade but with substantially less tail risk since both platforms will almost certainly resolve identically.
What Can Eat the Edge
Arbitrage looks mechanical on paper. In practice, four frictions kill most of the theoretical profit:
- Fees. Kalshi charges tiered fees up to 7% on winning positions in less liquid markets. Polymarket’s gas costs vary with Polygon network congestion. Model both before entry.
- Slippage. The quoted mid-prices above are from top-of-book. Working a size trade will move the price, especially on Kalshi where market depth is thinner for non-headline contracts.
- Resolution risk. Different platforms occasionally interpret the same real-world event differently. The Fed decision and ETF approval contracts have unambiguous resolution criteria; the Senate control and shutdown markets have slightly fuzzier definitions. Read the resolution rules on both sides before you leg in.
- Capital lockup. You post full collateral on both legs, so a 4-cent edge over three months is effectively a mid-single-digit return on capital. Compare against your opportunity cost before committing size.
Where to Trade
Both Kalshi and Polymarket offer the markets referenced above with real-money execution. New traders should fund small accounts on each before attempting any cross-platform arbitrage, since the mechanics of posting collateral and exiting positions differ meaningfully between the two.
Open an account on Kalshi for CFTC-regulated US access, or Polymarket for broader international coverage and crypto-native settlement. For a side-by-side on fees, markets, and limits, see our ranked guide to the best prediction markets.
Odds and spreads cited above reflect top-of-book pricing as of the morning of October 23, 2026, and will move throughout the trading day. This article is informational and not investment advice; all prediction market trading carries risk of loss.