Equus is a play by Peter Shaffer which tells the story of a psychiatrist who attempts to treat a young man who has a pathological fascination with horses.

Equus was originally staged by the National Theatre at the Old Vic in London in 1973; it was directed by John Dexter and starred Alec McCowen as psychiatrist Martin Dysart and Peter Firth as Alan Strang, the patient. It was subsequently presented on Broadway with Anthony Hopkins and Peter Firth. Later, Tom Hulce would play the role of Alan Strang and Richard Burton would replace Hopkins as Martin Dysart. The play received a Tony Award for best play in 1975.

Shaffer was inspired to write Equus when he heard of a crime involving a teenage boy's apparently senseless injury to horses. He set out to construct a fictional account of what might have caused the incident, without knowing any of the details of the crime. The play is essentially a detective story, with the psychiatrist trying to understand the cause of the boy's actions while wrestling with his own sense of purpose.

Equus was acclaimed not only for its well crafted plot and the performances by the actors, but also for its unusual staging. The scenes involving "horses" were portrayed by the main actors who were not in the particular scene. To play a horse, an actor donned a surreal-looking wire abstraction of a horse's head.

The play was made into a 1977 film which stars Richard Burton, Peter Firth, Colin Blakely , Joan Plowright and Jenny Agutter. The movie was adapted by Shaffer, and directed by Sidney Lumet. Unlike the play, the film version was placed in a "real" setting, and used real horses. Some critics and theatre purists found this objectionable, arguing that the spirit of the stage play was lost in the movie. Nonetheless, the movie was nominated for several Academy Awards: Best Actor in a Leading Role (Richard Burton), Best Actor in a Supporting Role (Peter Firth) and Best Writing, Screenplay Based on Material from Another Medium.