A B movie or B film is a low-budget commercial motion picture. In its original usage, during the Golden Age of Hollywood, the term more precisely identified films intended for distribution as the less-publicized bottom half of a double feature (akin to B-sides for recorded music). However, the US production of films intended as "second features" largely ceased by the end of the 1950s. With the emergence of commercial television at that time, film studio B movie production departments changed into television film production divisions. They created much of the same type of content in low-budget films and series. The term "B movie" continues to be used in its broader sense to this day. In post-Golden Age usage, B movies can range from lurid exploitation films to independent arthouse films.

In either usage, most B movies represent a particular genre—the Western was a Golden Age B movie staple, while low-budget science-fiction and horror films became more popular in the 1950s. Early B movies were often part of series in which the star repeatedly played the same character. Almost always shorter than the top-billed feature films, many had running times of 70 minutes or less. The term connoted a general perception that B movies were inferior to the more lavishly budgeted headliners; individual B films were often ignored by critics.

Latter-day B movies still sometimes inspire multiple sequels, but series are less common. As the average running time of top-of-the-line films increased, so did that of B pictures. In its current usage, the term has somewhat contradictory connotations: it may signal an opinion that a certain movie is (a) a "genre film" with minimal artistic ambitions or (b) a lively, energetic film uninhibited by the constraints imposed on more expensive projects and unburdened by the conventions of putatively serious independent film. The term is also now used loosely to refer to some higher-budget, mainstream films with exploitation-style content, usually in genres traditionally associated with the B movie.

From their beginnings to the present day, B movies have provided opportunities both for those coming up in the profession and others whose careers are waning. Celebrated filmmakers such as Anthony Mann and Jonathan Demme learned their craft in B movies. They are where actors such as John Wayne and Jack Nicholson first became established, and they have provided work for former A movie actors, such as Vincent Price and Karen Black. Some actors, such as Bela Lugosi, Eddie Constantine, Bruce Campbell, and Pam Grier, worked in B movies for most of their careers. The term "B actor" is sometimes used to refer to a performer who finds work primarily or exclusively in B pictures.