Off Broadway plays or musicals are performed in New York City in smaller theatres than Broadway, but larger than Off-Off-Broadway, productions.

Off Broadway theatres (venues) are those with 100 to 499 seats. The classification of theatres is governed by language in Actors' Equity contracts: a theater need not have a Broadway address, so long as it is "in the borough of Manhattan in any theater outside the area bounded by Fifth Avenue and Ninth Avenue from 34th Street to 56th Street, and by Fifth Avenue and the Hudson River from 56th Street to 72nd Street."

There was a time when, regardless of the size of the venue, a theatre could not be considered Off Broadway if it was within the "Broadway Box" (the traditional Broadway Theatre District). This is no longer the case as evidenced by a number of theatres in that area including, but not limited to New World Stages, The Little Shubert Theatre and The Snapple Center. However, if an Off Broadway theatre is located within the "Broadway Box" then there is a slightly higher minimum salary requirement for actors per their union, Actors' Equity.

Generally productions housed in Off Broadway theatres are less expensive, less publicized and less well-known. The smaller scale often allows more experimental, challenging work to be presented. Some groups which produce Off Broadway shows are non-profit rather than commercial producers, meaning they can more easily afford to take chances on plays which might not be commercial hits; however, they still have to ensure enough interest in their plays to have a large enough subscriber base to keep them financially sound. Some commercial productions have found a profitable niche in Off Broadway venues which allow long runs in their original theatres.

Occasionally, shows staged successfully Off Broadway will later have a run on Broadway. For instance, the musicals "A Chorus Line", Godspell, Avenue Q, Rent, Spring Awakening, Hair, Grey Gardens, Little Shop of Horrors, and Stephen Sondheim's Sunday in the Park with George and the plays Doubt and I Am My Own Wife were initially presented Off Broadway. However, some productions run successfully for several years in Off Broadway venues; examples are Stomp, Blue Man Group, Altar Boyz, A Perfect Crime and Naked Boys Singing. The Fantasticks, the longest-running musical in theatre history, spent its original 42-year run Off Broadway. In 2008, two Off-Broadway shows are transferring to Broadway, In the Heights and Passing Strange.

In London the closest equivalent to the term "Off Broadway" is known as fringe theatre, but it does not correspond to Off Broadway exactly because the structure of theatre in the two cities differs in many ways. In particular, there are no hard and fast divisions based on the number of seats in venues.