How Polymarket Works: On-Chain Prediction Markets Explained
A technical walkthrough of Polymarket's mechanics: USDC collateral on Polygon, conditional tokens, the order book, UMA oracle resolution, non-custodial design, and why US users are officially excluded.
Polymarket became the highest-profile on-chain prediction market largely because of the 2024 US election cycle, when its contracts attracted hundreds of millions in trading volume and significant mainstream media attention. Understanding how it actually works — at the level of smart contracts and token mechanics rather than the surface UI — reveals both why it is an interesting piece of financial infrastructure and why it carries risks that casual users often do not anticipate.
The Basic Setup: USDC on Polygon
Polymarket runs on the Polygon PoS network, not Ethereum mainnet. The choice is practical: Polygon transaction fees are a fraction of Ethereum’s, which matters when users are placing small trades and markets require frequent order updates.
Collateral is denominated in USDC, a dollar-pegged stablecoin issued by Circle. When you deposit to Polymarket, your USDC is locked in a smart contract and used to back the shares you buy or sell. All payouts are also in USDC. There is no native token you need to acquire; everything operates in dollar terms.
Because Polymarket is non-custodial, you connect with a self-custody wallet (commonly via Magic.link, which abstracts wallet creation into an email-based flow). Polymarket itself does not hold your funds — the smart contracts do. This is meaningfully different from a centralized exchange and has important implications for both security and regulatory treatment.
Conditional Tokens: The Share Structure
The share system Polymarket uses is built on the Conditional Token Framework (CTF), originally developed by Gnosis. Each market has two or more outcome tokens. In a binary yes/no market:
- A YES token pays $1 USDC if the event resolves YES and $0 otherwise.
- A NO token pays $1 USDC if the event resolves NO and $0 otherwise.
A complete set of outcome tokens always sums to $1. This means you can mint a complete set by depositing $1 USDC and receiving one YES token and one NO token. Alternatively, you can redeem a complete set back for $1 USDC at any time. This arbitrage relationship anchors prices — if YES trades at $0.70 and NO trades at $0.35, someone can profitably mint and sell, pushing prices back toward sum-to-one.
When a market resolves, the winning token becomes redeemable for $1 USDC and the losing token becomes worthless. Holders of losing shares receive nothing.
The Order Book
Polymarket uses a central limit order book (CLOB), not an automated market maker. This means:
- Buyers post bids at prices they are willing to pay.
- Sellers post asks at prices they are willing to accept.
- When a bid meets or exceeds an ask, a trade executes.
The order book is operated off-chain for speed (on-chain order books are slow and expensive), but settlement — the actual transfer of tokens — happens on-chain. This hybrid design is common in decentralized finance. The trade-off is that Polymarket has meaningful centralized components despite the on-chain settlement layer.
Spreads vary significantly by market. A major active market might have a bid-ask spread of one or two cents on a $0.50 share. A thin market might have spreads of five to fifteen cents, which represents a significant drag on returns. If you buy YES at $0.55 and the true probability is $0.50, you need the event to be more likely than the market believes just to break even.
UMA Protocol: The Oracle Resolution Layer
The most consequential — and most dispute-prone — part of Polymarket is how markets resolve. The platform uses UMA Protocol’s optimistic oracle system.
The process works roughly like this:
- When an event occurs, anyone can submit a proposed resolution (for example, “YES, the bill passed”).
- The proposed resolution is posted to UMA’s smart contracts with a bond. If nobody disputes it within the challenge window (typically two hours), the resolution is accepted and the market settles.
- If someone does dispute, UMA token (UMA) holders vote on the correct resolution. The dispute bond from the losing side is paid to the winning side.
This design is clever: it assumes most resolutions are uncontroversial and handles them cheaply, reserving the expensive on-chain voting for genuine disputes. In practice, the vast majority of markets settle without dispute.
When disputes do arise, they can be prolonged and unpredictable. Ambiguously worded markets have resolved in ways that surprised many participants — including cases where a technically correct resolution felt wrong given traders’ reasonable expectations. The resolution rules are determined by the market question text at creation, so question wording matters enormously.
Polymarket itself can intervene in some cases, but the platform has attempted to move toward UMA-governed resolution to reduce its own centralized authority over outcomes.
Non-Custodial Design and What It Means
Because funds are held in smart contracts rather than on Polymarket’s balance sheet, several things follow:
You bear smart contract risk. A bug in the CTF contracts or Polymarket’s order book contracts could result in lost funds. These contracts have been audited, but audits do not guarantee safety.
Polymarket cannot easily freeze or return funds. If you lose access to your wallet, there is no customer support that can recover it. If a market resolves incorrectly in your view, there is no chargeback.
Regulatory intervention is harder but not impossible. Regulators cannot simply demand that Polymarket return funds the way they could with a centralized exchange. However, they can pursue the people running the platform and block access at the infrastructure level.
The non-custodial design is also why Polymarket is more resistant to outright fraud than a custodial platform — the operator cannot simply abscond with user funds. But it substitutes smart contract risk and oracle risk in place of counterparty risk.
US Access Restrictions
Polymarket officially blocks US users and users from several other jurisdictions. This is enforced primarily through IP geolocation. Users who circumvent these restrictions via VPNs are violating Polymarket’s terms of service and potentially applicable law.
In 2022, Polymarket settled with the US Commodity Futures Trading Commission (CFTC) for $1.4 million over charges that it had offered event contracts to US users without proper authorization. The settlement required Polymarket to block US access going forward. The legal question of whether on-chain prediction markets constitute regulated financial products in the US remains contested and is discussed further in our article on regulation and legal frameworks.
The CFTC settlement did not resolve the underlying question of legality — it simply established that Polymarket specifically was operating improperly during that period without authorization. Kalshi, by contrast, pursued and obtained CFTC authorization, which is why it can legally serve US users.
Fees
Polymarket charges no fees on trades directly. Revenue comes from a small take on market creation and some spread-related mechanics. However, the effective cost of trading includes:
- The bid-ask spread on each trade
- Polygon gas fees (small but nonzero)
- Any USDC bridging fees if moving funds from Ethereum mainnet
For small trades, gas costs are negligible. For large trades on thin markets, the spread can be the dominant cost.
Summary
Polymarket is a genuinely novel piece of financial infrastructure: an order book prediction market that settles on a public blockchain, uses a decentralized oracle for resolution, and holds user funds in smart contracts rather than corporate accounts. These properties make it meaningfully different from both traditional gambling sites and centralized exchanges.
They also create a distinct risk profile. Smart contract vulnerabilities, oracle disputes, thin market spreads, and regulatory uncertainty all represent real exposure for participants. Understanding how these mechanics work is a prerequisite for participating responsibly.
For context on the regulatory side, see our coverage of Kalshi and regulated event contracts, or read the broader prediction market risks article.