Sir Derek George Jacobi, (; born 22 October 1938) is an English actor and stage director.

A "forceful, commanding stage presence", Jacobi has enjoyed a successful stage career, appearing in such stage productions as Hamlet, Uncle Vanya, and Oedipus the King. He has twice been awarded a Laurence Olivier Award, first for his performance of the eponymous hero in Cyrano de Bergerac in 1983 and the second for his Malvolio in Twelfth Night in 2009. He also received a Tony Award for his performance in Much Ado About Nothing in 1984 and a Primetime Emmy Award in 1988 for The Tenth Man. His stage work includes playing Octavius Caesar, Edward II, Richard III and Thomas Becket.

In addition to being a founder member of the Royal National Theatre and winning several prestigious theatre awards, Jacobi has also enjoyed a successful television career, starring in the critically praised adaptation of Robert Graves's I, Claudius (1976), for which he won a BAFTA; in the titular role in the medieval drama series Cadfael (1994–1998), as Stanley Baldwin in The Gathering Storm (2002), as Stuart Bixby in the ITV comedy Vicious (2013–2016) and as Alan Buttershaw in Last Tango in Halifax (2012–2016). Jacobi also portrayed a version of The Master in the long running science fiction series Doctor Who.

Though principally a stage actor, Jacobi has appeared in a number of films, including The Day of the Jackal (1973), Henry V (1989), Dead Again (1991), Gladiator (2000), Gosford Park (2001), The Riddle (2007), The King's Speech (2010), My Week with Marilyn (2011), Cinderella (2015); and Murder on the Orient Express (2017).

He was knighted in 1994 and has also been made a member of the Danish Order of the Dannebrog.