Off-Broadway theatre is a term for a professional venue in New York City with a seating capacity between 100 and 499, and for a specific production of a play, musical or revue that appears in such a venue, and which adheres to related trade union and other contracts. These theatres are smaller than Broadway theatres.

Originally referring to the location of a venue and its productions on a street intersecting Broadway in Manhattan's Theatre District, the hub of the theatre industry in the United States, the term later became defined by the League of Off-Broadway Theatres and Producers as a professional venue in New York City with a seating capacity between 100 and 499, or a specific production that appears in such a venue, and which adheres to related trade union and other contracts.

Previously, regardless of the size of the venue, a theatre was not considered Off-Broadway if it was within the "Broadway Box" (the traditional Broadway Theatre District, extending from 40th to 54th Street, and from west of Sixth Avenue to east of Eighth Avenue, and including Times Square and 42nd Street). The contractual definition changed this to encompass theatres meeting the standard, which is beneficial to these theatres because of the lower minimum required salary for Actors' Equity performers at Off-Broadway theatres as compared with the salary requirements of the union for Broadway theatres. Examples of Off-Broadway theatres within the Broadway Box are New World Stages, the Little Shubert Theatre and the Snapple Theater Center.