Prediction Market Arbitrage: Finding and Trading Mispricings

In financial markets, arbitrage is a risk-free profit from simultaneously buying and selling the same asset at different prices. In prediction markets, pure arbitrage is rare — but near-arbitrage and statistical arbitrage opportunities appear regularly.

The first type is the most reliable but increasingly rare as platforms grow more efficient. The second appears regularly if you know what to look for. The third is the largest opportunity by volume.

Within Polymarket, several types of logical constraints should hold between related markets:

Temporal ordering — "Will X happen by March?" must be priced at or below "Will X happen by June?" since everything that happens by March also happens by June. When this is violated, buy the longer-dated market and sell (short via NO) the shorter-dated market.

Mutually exclusive outcomes — "Will Democrat win?" + "Will Republican win?" + "Will Third Party win?" should sum to approximately 100¢. When they sum to 105¢+, buying NO on all outcomes and holding until one resolves is positive expected value.

Logical implication — "Will A happen?" and "Will A and B both happen?" The joint probability can never exceed the individual probability. "Will Biden run AND win?" should be lower than "Will Biden run?"

Scanning for these violations systematically, especially right after major news events when markets update asynchronously, is one of the most reliable arbitrage strategies.

Polymarket and Kalshi often list markets on the same underlying event with different prices. When the same question trades at 65¢ on one platform and 60¢ on another, buying the cheap side and selling the expensive side creates a locked-in profit.

In practice, cross-platform arb is most available immediately after a new market launches on one platform before participants on both platforms have noticed the discrepancy. Monitoring both platforms' new market listings is the key to finding these opportunities first.

Pure arbitrage is hard to find and quickly disappears. Correlated market plays — where two logically related markets are mispriced relative to each other — are both more common and more scalable.

Example: In 2024, the "Democrats win the House" market and the "Biden approval rating above 45% on election day" market were logically correlated. One could be used to approximate the probability range for the other using a simple conditional probability model. When the two markets diverged from their expected relationship, it was a tradeable opportunity.

When your correlation model shows two markets pricing an impossible or highly unlikely relationship, trade the mispriced side.

Prediction market arbitrage opportunities have a half-life. The moment a mispricing becomes visible, participants who trade on it push prices toward equilibrium. High-profile arb opportunities (large price differences, major markets) disappear within hours. Subtle ones can persist for days.

The arbitrage mindset — systematically checking for logical consistency across markets — is one of the highest-value skills in prediction market trading, even when pure arb opportunities aren't available.