Billion Dollar Brain is a 1967 British espionage film directed by Ken Russell and based on the 1966 novel Billion-Dollar Brain by Len Deighton. The film features Michael Caine as secret agent Harry Palmer, the anti-hero protagonist. The "brain" of the title is a sophisticated computer with which an anti-communist organisation controls its worldwide anti-Soviet spy network.

Billion Dollar Brain is the third of the Harry Palmer film series, preceded by The Ipcress File (1965) and Funeral in Berlin (1966). It is the only film in which Ken Russell worked as a mainstream 'director-for-hire', and the last film of Françoise Dorléac. A fourth film in the series, an adaptation of Horse Under Water, also to be released by United Artists, was tentatively planned but never made. Caine played Palmer in two later films, Bullet to Beijing and Midnight in Saint Petersburg.