A Multiple Listing Service (MLS) (also Multiple Listing System or Multiple Listings Service) is a group of private databases 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. There is no single authoritative "MLS", and no universal data format. The many local and private databases--some of which are controlled by single associations of realtors or groupings of associations (which represent all brokers within a given community or geographical area) or by real estate brokers--are collectively referred to as the MLS because of their reciprocal access agreements.

Seen most widely in the US and Canada but spreading to other countries in a variety of forms, 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 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, s/he 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.

In North America, 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, but 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 of the MLS is generally considered to be essential to the practice of real estate brokerage.