Phase IV is a 1974 science-fiction horror film directed by graphic designer and filmmaker Saul Bass, and written by Mayo Simon, inspired by H. G. Wells's 1905 short story "Empire of the Ants". The film stars Michael Murphy, Nigel Davenport and Lynne Frederick.

Interiors were shot at Pinewood Studios in England and exterior locations were shot in Kenya, though the film is set in the Arizona desert in the United States. It was produced by Alced Productions and Paramount Pictures.

The film was a box office flop and thus the only feature film directed by Bass. It has since gained a cult following, due to TV airings beginning in 1975 and also being shown on Mystery Science Theater 3000 during the KTMA era.

A novelization of the script, written by Barry N. Malzberg, was published as Phase IV in November 1973.