Polymarket & Kalshi Odds Data for Developers | Odds API

The prediction market boom isn't slowing down. In 2025, Polymarket and Kalshi combined for over $40 billion in trading volume. Academic research found that arbitrage bots extracted more than $40 million in profit from pricing inefficiencies alone. And those inefficiencies? Still wide open.

We've added Polymarket and Kalshi to Odds API, giving you everything you need to tap into this opportunity.

What Are Prediction Markets?

If you're new to prediction markets, here's the quick version: they're exchanges where you bet on real-world outcomes. Instead of a bookmaker setting odds, the market price reflects what traders collectively believe will happen.

Polymarket is crypto-native. It runs on the Polygon blockchain, settles in USDC, and is accessible globally (though US users face restrictions). It gained massive attention during the 2024 US election when it processed $3.7 billion in bets on the presidential race alone.

Kalshi is the regulated alternative. It's a CFTC-approved designated contract market based in the US. You trade in USD, not crypto, and it requires identity verification for full access. In 2025, Kalshi pivoted heavily into sports markets - now accounting for roughly 75% of its volume.

Both platforms let you buy "Yes" or "No" shares on outcomes. If you're right, each share pays out $1. If you're wrong, it's worth nothing. The current price represents the market's implied probability.

Polymarket vs Kalshi: Key Differences

Here's what matters: these platforms don't always agree on price. The same event can trade at different odds on each platform - and that's where the opportunity lies.

What's Included in Our Feed

Our prediction market data gives you:

• Top-of-book prices with depth: Know how much liquidity sits at each price level
• Lay odds: Back and lay on every market (bet for or against outcomes)
• 1-second latency: Fresh data for time-sensitive strategies
• Native bookmaker IDs: Kalshi event IDs (like KXEPLGAME-26JAN31LEEARS) and Polymarket market IDs (like 170912) so you can plug straight into their execution APIs

That last point matters. We return the platform's native identifiers, so you don't need mapping tables. You can route directly to their APIs for execution. Check our API documentation for implementation details.

Why Depth Changes Everything

Here's a real example. Leeds vs Arsenal, January 2025:

Kalshi:

• Leeds to win: 6.25 odds
• Depth at best price: $9,215

Polymarket:

• Leeds to win: 6.67 odds
• Depth at best price: $12,128

Polymarket has more liquidity but a slightly worse price. Now look at the lay side - if you want to bet against Leeds winning:

• Kalshi: $5,400 available at 1.61
• Polymarket: $9,400 available at 1.61

Same lay price, but Polymarket has nearly double the liquidity. If you're placing a large order against Leeds, you'd route to Polymarket to minimise slippage. If you're backing Leeds and want the best price, Kalshi wins.

Knowing where the depth sits determines which platform you route your order to.

The Arbitrage Opportunity

Prediction markets are still young, and price discrepancies between platforms are common. Kalshi and Polymarket don't always agree with each other - and neither perfectly tracks the traditional sportsbook market.

Academic research from IMDEA Networks Institute analysed 86 million bets placed between April 2024 and April 2025. They found:

• $40+ million in arbitrage profits extracted by sophisticated traders
• The top three wallets placed over 10,200 bets and made $4.2 million combined
• Opportunities existed in over 7,000 markets with measurable mispricing
• Politics markets were the most profitable; sports markets had the most frequent (but smaller) opportunities

Here's how it works. Sometimes the "Yes" and "No" prices on a single market don't add up to $1.00. When that happens, you can buy both sides and lock in a guaranteed profit regardless of the outcome. Cross-platform arbitrage works similarly - when Polymarket prices an event differently than Kalshi, there's an opportunity to bet both sides across venues.

If you're building arbitrage strategies, market-making bots, or cross-platform trading systems, this is exactly the data you need.

Who This Is For

Arbitrage traders: Monitor price discrepancies across Polymarket, Kalshi, and traditional sportsbooks. Execute when spreads exceed your threshold.

Market makers: Provide liquidity and profit from bid-ask spreads. Prediction markets still lack the institutional market-making infrastructure of traditional exchanges.

Quantitative traders: Build models that identify mispriced contracts based on news flow, related market movements, or cross-platform signals.

Sports betting developers: Add prediction market odds alongside traditional bookmaker data. Your users get a fuller picture of where value exists.

Same Pricing, No Add-Ons

Prediction market data is included in your existing Odds API plan. No upgrades required.

Get Started

Ready to build? Check out our Prediction Markets Guide for integration details, endpoint documentation, and example code.

The same data that powered $40 million in arbitrage profits is now in your API.