Guide
Polymarket US arbitrage: what's possible and how
Polymarket US is the US version of Polymarket, run as a separate exchange from the international site. That opens two kinds of spreads for US traders: gaps between Kalshi and Polymarket US, and bundles inside Polymarket US itself.
Polymarket US vs Polymarket
They're separate exchanges with separate order books and prices. The international site at polymarket.com isn't available to US users. Polymarket US is the version built for them, with its own app and API. Because the books are separate, the same event can trade at different prices on each, and on Kalshi.
Two kinds of spreads
1. Kalshi + Polymarket US pairs
Buy YES on one exchange and NO on the other when the two cost less than $1 after both platforms' fees. Both are US exchanges, so a US trader can place both sides.
2. Bundles inside Polymarket US
When an event has several outcomes and exactly one must win (for example a game result), buying YES on every outcome pays exactly $1. If all of them together cost less than $1 after fees, the gap is locked in.
Fees decide most trades
Polymarket US charges takers θ × contracts × P × (1 − P), where θ is set per market (0.0695 on most markets as of October 2026). Kalshi charges 0.07 × contracts × P × (1 − P), rounded up to the cent. When both sides charge fees, a gap that looks good can disappear:
| 100 contracts each | Prices add up to | Fees | Profit |
|---|---|---|---|
| Kalshi YES 46¢ + Polymarket US NO 51¢ | 97¢ | $3.48 | -$0.48 |
| Kalshi YES 45¢ + Polymarket US NO 51¢ | 96¢ | $3.48 | $0.52 |
Illustrative prices. Try your own in the arbitrage calculator.
A 3¢ gap loses money here; it takes about 4¢ to clear both fees near even odds.
Placing both sides fast
Both Kalshi and Polymarket US have APIs, so a tool can place both sides for you. On ArbTrader's Quant plan, one tap sends both orders: each fills completely at your price or not at all, the side with the least for sale goes first, and if one side still misses it either completes the trade at break-even or sells back what filled. See how to connect your accounts.
Common questions
Can US users trade Polymarket arbitrage?
Not on the international Polymarket site, which isn't available in the US. US users can trade gaps between Kalshi and Polymarket US, and bundles inside Polymarket US, subject to each platform's eligibility rules and their state.
What are Polymarket US fees?
Takers pay theta times contracts times P times (1 minus P), with theta set per market (0.0695 on most markets as of October 2026). Check the market's own fee before trading.
ArbTrader scans 30,000+ Kalshi, Polymarket and Polymarket US events about every 15 seconds and does this math for you.
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