Polymarket & Kalshi Historical Data: Free Sources & APIs

Whether you're backtesting a strategy, doing academic research on forecast accuracy, or building charts, you need historical prediction market data — and it's scattered across more places than you'd expect. Here's every meaningful source for Polymarket and Kalshi historical data, what each covers, and what it costs (mostly: nothing).

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Polymarket historical data

Polymarket's read APIs are free and require no authentication. Three endpoints matter for history:

1. Price history — CLOB API

GET https://clob.polymarket.com/prices-history returns time-series prices for a specific CLOB token (each market has a Yes token and a No token):

import requests

resp = requests.get(
    "https://clob.polymarket.com/prices-history",
    params={"market": "YOUR_CLOB_TOKEN_ID", "interval": "max"},
)
history = resp.json()["history"]  # [{"t": timestamp, "p": price}, ...]

Get the token ID from the market object's clobTokenIds field via the Gamma API (gamma-api.polymarket.com/markets). Note it's a JSON-encoded string you must parse. For a full walkthrough of Gamma, CLOB, and the Data API, see our complete Polymarket API guide.

2. Trade history — Data API

GET https://data-api.polymarket.com/trades returns individual trades — the granular record of who traded what, when, at what price. Essential if candle-level data isn't enough (e.g., slippage modeling).

3. On-chain data — Polygon

Every Polymarket trade settles on Polygon, so the blockchain itself is a complete, tamper-proof historical record. Querying it directly (via a Polygon node or an indexer) is the most work, but it's the ground truth — useful for research that needs wallet-level attribution, like tracking historical smart-money performance.

Resolved markets: the Gamma API keeps closed markets queryable (closed=true), so you can enumerate historical questions and outcomes for calibration studies.

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Kalshi historical data

Kalshi's market data endpoints are also public and free. The wrinkle: Kalshi splits data into live and historical databases, with a cutoff timestamp separating them.

1. Candlesticks — live markets

GET /markets/{ticker}/candlesticks on api.elections.kalshi.com/trade-api/v2 returns OHLC data at 1-minute, 1-hour, or 1-day resolution. There's also a batch endpoint for multiple markets, and event-level candlesticks aggregated across an event's markets.

2. The /historical/ endpoints — archived markets

Markets settled before the historical cutoff move to dedicated endpoints: GET /historical/markets, GET /historical/markets/{ticker}/candlesticks, and GET /historical/trades. If a market you're looking for 404s on the live endpoints, it's in the archive. GET /historical/cutoff_timestamps tells you where the boundary sits.

3. Public trades

GET /markets/trades returns the tape — every public trade with price, quantity, and timestamp, cursor-paginated:

import requests

resp = requests.get(
    "https://api.elections.kalshi.com/trade-api/v2/markets/trades",
    params={"ticker": "CONTROLH-2026-D", "limit": 100},
)
for t in resp.json()["trades"]:
    print(t["created_time"], t["yes_price"], t["count"])

Prices are in cents (1–99), mapping directly to implied probability. New to the Kalshi API? Start with our getting started guide.

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The gap: cross-platform history

Here's what none of the above gives you: the same event's price history across platforms, aligned.

For most interesting questions — Was Kalshi or Polymarket faster to react to the jobs report? How large do cross-platform spreads get before they close? Which platform's prices are better calibrated? — you need matched, time-aligned series from multiple venues. Building that yourself means running collectors against every platform's API, 24/7, plus solving entity resolution (Kalshi's PRES-2028-DEM = Polymarket's presidential-2028 = PredictIt's #8102).

That matched, multi-platform view is what the Prediction HuntPrediction.com API provides: 6 platforms behind one key, with events automatically resolved to a single entity across venues — so a single query returns each platform's pricing for the same real-world question:

curl -H "X-API-Key: YOUR_API_KEY" \
  "https://prediction.com/api/v2/search?q=fed+rate+hike"

For ongoing collection, WebSocket price channels let you build your own tick archive across all platforms from one connection instead of six. The free tier (1,000 requests/month) covers exploratory research; Dev covers production collection.

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Which source should you use?

You need	Use
One Polymarket market's price chart	CLOB /prices-history
Polymarket trade-level data	Data API /trades
Wallet-level attribution	Polygon on-chain data
Kalshi OHLC for a live market	/markets/{ticker}/candlesticks
Kalshi data on settled/old markets	/historical/ endpoints
Same event across platforms, matched	Prediction HuntPrediction.com API
Cross-platform tick archive going forward	Prediction HuntPrediction.com WebSockets

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Researching the platforms themselves rather than their data? Our Polymarket vs Kalshi side-by-side comparison covers fees, liquidity, and market selection.

Backtest across every platform, not just one. Get a free Prediction HuntPrediction.com API key — matched cross-platform market data in a single call.