Is Arbitrage Betting Legal? What Prediction-Market Traders Need to Know (2026)

Arbitrage itself is not illegal in the United States. It is a trading strategy: buying opposite outcomes of the same event across two exchanges at a combined price below the guaranteed payout. The real question is not whether arbitrage is allowed; it is whether you are allowed to hold an account on each platform the trade touches.

Arbitrage itself is a trading strategy, not a crime

There is nothing in the mechanism of arbitrage that breaks a law. You place a bid at the listed price on one exchange, a bid at the listed price on another, and you wait for the market to settle. Kalshi and Polymarket both structure these trades as event contracts, instruments that pay $1 or $0 depending on whether a specified event happens. Buying an event contract at the listed price is an ordinary trade executed through the exchange's normal order flow. No law in the United States prohibits buying two event contracts that happen to reference the same outcome.

This is exactly how the scanner works: ArbLense compares live prices for the same event across venues and flags the moments where the combined cost of YES and NO falls under a dollar. The scanner does not execute trades or custody funds. It surfaces the opportunity with the return and risk already worked out, and you open each venue and place both orders yourself.

The confusion around legality mostly comes from a different industry entirely.

Sportsbooks vs prediction markets: why the difference matters

Traditional sportsbooks have a direct financial reason to stop arbitrage. A sportsbook holds the other side of your bet, so a guaranteed win for you is a guaranteed loss for the house, and many sportsbook terms of service reserve the right to limit stakes or close accounts flagged for "advantage play," arbing included. That is a business decision written into a private contract, not a criminal statute.

Prediction-market exchanges are structured differently. Kalshi, Polymarket, and Opinion match buyers and sellers and collect a fee per trade, rather than taking the other side of the bet themselves. An exchange earns from volume regardless of the outcome, so there is no equivalent incentive to ban prediction market arbitrage the way a sportsbook bans arbing. (A separate question people search, "is Kalshi gambling," is really about how sports contracts get classified, not about arbitrage, and it deserves its own page rather than a detour here.)

The structural difference in one pass:

• Counterparty. At a sportsbook, the book itself holds the other side of your bet. On a prediction-market exchange, your counterparty is another trader, matched by the exchange.
• Revenue model. A sportsbook earns from the vig built into both sides of the line. An exchange earns a per-trade fee and is paid on volume either way.
• Stance on arbitrage. Sportsbooks often limit or close accounts flagged for arbing. Exchanges have no structural incentive to restrict it.

What the platform rules actually say

Kalshi is registered with the CFTC as a designated contract market, the same regulatory category as exchanges like the CME. Its live Member Agreement defines "Event Contracts" as any contract listed on the exchange based on the outcome of an event, and it sets out a list of Restricted Jurisdictions where trading those contracts is barred, plus broad discretion for Kalshi to suspend or condition membership. Nowhere in that document does the word arbitrage appear, and nothing in it singles out buying opposite outcomes across two exchanges as a violation. That absence is worth stating plainly: it is a finding, not a gap in this research.

So, is Polymarket arbitrage legal? Polymarket's Market Integrity page, which explains what its Terms of Use prohibit, lists insider trading, fraud, market manipulation, self-dealing, front-running, wash trading, spoofing, and similar conduct as violations. Arbitrage is not on that list. Polymarket also restricts access by jurisdiction rather than by strategy, and using a VPN to route around a geo-block is itself a terms violation that can get an account frozen, separate from anything about arbitrage.

Opinion is the third venue the ArbLense scanner covers. Its terms of service page loads through the trading app itself rather than as a plain page, so the text could not be reviewed directly for this article the way Kalshi's and Polymarket's documents were. Treat that as an open item, not a confirmed finding, and check Opinion's current terms directly before trading there.

Traders who layer automation on top of manual routes encounter a separate set of rules governing bots and API access, which is a separate topic from plain arbitrage and belongs in its own guide.

If you want to see how the arbitrage scanner compares prices across all three venues in real time, that is the core product. Some routes run beyond that coverage: cross-venue arbitrage routes between Polymarket and Betfair are a worked example.

Access is the real legal question

None of the three venues make arbitrage itself the issue. Jurisdiction does. Whether you can even open an account, fund it, and hold a position on Kalshi or Polymarket depends on where you live, and that picture is currently a patchwork of federal approval layered against a handful of state-level challenges. A full state-by-state breakdown, including the Kalshi-Polymarket arbitrage angle specifically, is outside the scope of this piece; it belongs on dedicated legality pages for each platform.

Taxes in one paragraph

Profit from arbitrage trades is taxable income in the US, whether it comes from one leg winning and the payout exceeding what you spent on both legs, or from any other gain the IRS would treat as income. Reporting requirements depend on your personal situation and how each platform classifies the payout. This is not tax advice. Talk to an accountant before you assume anything about how a specific trade will be treated.

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