Multiple Listing Service (MLS) (also Multiple Listing System or Multiple Listings Service) is a database which allows real estate brokers representing sellers under a listing contract to widely share information about properties with real estate brokers who may represent potential buyers or wish to cooperate with a seller's broker in finding a buyer for the property.

The purpose of the MLS is to enable the efficient distribution of information so that, when a real estate agent is introduced to a potential home buyer, he/she may search the MLS system and retrieve information about all homes for sale in a given area or price range, whether under a listing contract by that agent's brokerage or by all participating brokers. The MLS combines the listings of all available properties that are represented by brokers who are both members of that MLS system and of NAR or CREA, (the National Association of Realtors in the US or the Canadian Real Estate Association).

The MLS systems are governed by private entities, and the rules are set by those entities with no state or federal oversight, beyond any individual state rules regarding real estate. MLS systems set their own rules for membership, access, and sharing of information, and are subject to nationwide rules laid down by NAR or CREA. An MLS may be owned and operated by a real estate company, a county or regional real estate Board of REALTORS or Association of REALTORS, or by a trade association. Membership to the MLS is generally considered to be essential to the practice of real estate brokerage.

Most MLS systems restrict membership and access to real estate brokers (and their agents) who are appropriately licensed by the state (or province); are members of a local Board or Association of REALTORS; and are members of the trade association (e.g., NAR or CREA).

A person selling his/her own home (i.e., acting as a for sale by owner) cannot put a listing for the home directly into the MLS. Similarly, a properly licensed broker who chooses to neither join the trade association nor operate a business within the associations's rules, cannot join the MLS.