Sir Derek George Jacobi KBE, (born October 22, 1938), is a British actor, knighted in 1994 for his services to the theatre.

Jacobi was born in Leytonstone in east London; he studied at the University of Cambridge before embarking on his stage career. He quickly came to the fore, and his talent was recognised by Laurence Olivier, who gave him the role of Cassio in his 1965 film of Othello. Although Jacobi's name was becoming known and he was increasingly busy with stage and screen acting, his big breakthrough did not come for another ten years. It was the title role of the BBC's blockbuster series I, Claudius (1976) that finally cemented his popular reputation. His performance as the stammering, twitching Emperor Claudius won him many plaudits. It was a performance he would later come close to repeating in the role of Alan Turing in the stage play, Breaking the Code (of which a televised version was made in 1996).

Jacobi continued to play Shakespeare, notably in Kenneth Branagh's 1989 film of Henry V (as the Chorus) and in two notable productions of Hamlet (as Hamlet in the BBC Television Shakespeare production in 1980 and as Claudius in Kenneth Branagh's movie in 1996), but also attracted new audiences as the detective-monk Cadfael in ITV's adaptation of the novels of Ellis Peters. Jacobi has also done the narration for an audio book version of the Iliad. He remains one of Britain's foremost actors.

In the early 2000s Jacobi toured internationally for a production of The Hollow Crown with Sir Donald Sinden , Commander Ian Richardson and Dame Diana Rigg.

In 2004, Jacobi starred in Don Carlos at the Crucible Theatre in Sheffield, in an acclaimed production, which opens at the Gielgud Theatre in London in late January 2005.

In 2005, he will play the role of Alexander Corvinus in the movie Underworld: Evolution