Alternative trading system (ATS) is a US and Canadian regulatory term for a non-exchange trading venue that matches buyers and sellers to find counterparties for transactions. Alternative trading systems are typically regulated as broker-dealers rather than as exchanges (although an alternative trading system can apply to be regulated as a securities exchange).

In general, for regulatory purposes, an alternative trading system is an organization or system that provides or maintains a market place or facilities for bringing together purchasers and sellers of securities, but does not set rules for subscribers (other than rules for the conduct of subscribers trading on the system). An ATS must be approved by the United States Securities and Exchange Commission (SEC) and is an alternative to a traditional stock exchange. The equivalent term under European legislation is a multilateral trading facility (MTF).

These venues play an important role in public markets for allowing alternative means of accessing liquidity. They can be used for trading large blocks of shares away from the normal exchange, a practice that could otherwise skew the market price in a particular direction, depending on a security's market capitalization and trading volume. ATSs are generally electronic but don't have to be. ATSs can be distinguished from electronic communication networks (ECNs), which are a "fully electronic subset of ATSs that automatically and anonymously match orders".

In recent years, the SEC and other regulators have ramped up their enforcement activities with respect to alternative trading systems, initiating broad investigations and bringing actions for various violations, such as trading against customer order flow or permitting external vendors to retain and make use of confidential customer trading information in the vendor’s trading activities.

Regulation ATS was introduced by the SEC in 1998 and is designed to protect investors and resolve any concerns arising from this type of trading system. Regulation ATS requires stricter record keeping and demands more intensive reporting on issues such as transparency once the system reaches more than 5% of the trading volume for any given security.