Polymarket US API: what it exposes, how it authenticates, where it stops

What the API exposes

The authenticated API groups into Orders (create, modify, cancel, cancel all, preview, close position, plus batch create, cancel and modify), Portfolio (positions, activities) and Account (balances and buying power). Combos and RFQ endpoints let participants build multi-leg instruments and quote on them.

The public API needs no key: markets with order books, best bid and offer, settlement prices and price history; events; series; sports leagues, teams and schedules; tags; incentive programs; and full-text search.

• wss://api.polymarket.us/v1/ws/private streams orders (type 1), positions (3) and account balance (4).
• wss://api.polymarket.us/v1/ws/markets streams full market data (1), a lite price feed (2) and trades (3). Both sockets require API-key headers on the handshake.
• Listed markets today: professional football, basketball, hockey and college football, with more described as coming soon.

Authentication and rate limits

Keys come from polymarket.us/developer after you create an account in the iOS app and pass identity verification. You get a Key ID and a Secret Key shown once. Each request carries X-PM-Access-Key, X-PM-Timestamp (milliseconds, within 30 seconds of server time) and X-PM-Signature, an Ed25519 signature over timestamp + method + path, base64 encoded.

The limit is 20 requests per second per API key across authenticated endpoints and 20 per second per IP on public endpoints, returning 429 when exceeded. A separate 5-second latency stopgap rejects orders not processed in time with the message Global Rate Limit Exceeded; the docs say not to throttle in response.

Pricing and access

Fees effective July 1, 2026: Fee = theta x C x p x (1 - p) with taker theta 0.06 (maximum $1.50 per 100 contracts at $0.50) and a maker rebate of 0.0125 (maximum $0.31). Taker rebates of 10%, 25% and 50% apply above $250,000, $1,000,000 and $10,000,000 of prior-month taker volume. Withdrawals are free.

SDKs: pip install polymarket-us (Python 3.10+) and npm install polymarket-us (Node 18+), with GitHub repos under the Polymarket organization. The docs say to email support if you need an invite code for the app.

Where it falls short

20 requests per second is the lowest published ceiling among the venue APIs on this site, and the docs push you to WebSocket subscriptions instead of polling.

You must install the iOS app and clear identity verification before any key exists, and switching sign-in methods (Apple, Google, email) can break key access.

No sandbox or paper environment appears in the docs, and the market list is sports only for now.

Who should use it

US traders who want a regulated, dollar-denominated book and prefer key signing over wallet signing. The typed SDKs cover every endpoint group including both sockets.

Non-US developers and anyone needing politics or economics markets should look at the international Polymarket API or Kalshi.