Defence of the Realm is a 1986 British political thriller film directed by David Drury, starring Gabriel Byrne, Greta Scacchi, and Denholm Elliott, with Robbie Coltrane in a supporting role.

The film takes its title from the Defence of the Realm Act 1914, passed in the United Kingdom at the start of the First World War, which gave the government wide-ranging powers during the war.

It was shot at Shepperton Studios and on location in London and Duxford in Cambridgeshire. The film's sets were designed by the art director Roger Murray-Leach. The film was distributed in the United Kingdom by the Rank Organisation, one of the last films to be released by the company.

The plot mirrors the real-life British spy scandal known as the Profumo affair.