Ivan Romilly Moffat (18 February 1918 – 4 July 2002) was a British screenwriter, film producer and socialite who, with Fred Guiol, was nominated for an Academy Award for Best Adapted Screenplay for adapting Edna Ferber's eponymous novel into the film Giant (1956). Moffat was the grandson of actor-manager Sir Herbert Beerbohm Tree.
After studying at the London School of Economics, Moffat became a socialite and began to make films to promote the war effort. During World War II he filmed activities of the US Army, meeting director George Stevens, whom he soon followed to Hollywood and assisted at Paramount Pictures. In the 1950s, between his two marriages, Moffat had a string of love affairs, notably with Elizabeth Taylor and Lady Caroline Blackwood. Beginning in 1956 he wrote or co-wrote screenplays for a number of well-known films, in addition to Giant, and in the 1970s wrote for television.