Frederick Lonsdale (5 February 1881 – 4 April 1954) was a British playwright known for his librettos to several successful musicals early in the 20th century, including King of Cadonia (1908), The Balkan Princess (1910), Betty (1915), The Maid of the Mountains (1917), Monsieur Beaucaire (1919) and Madame Pompadour (1923). He also wrote comedy plays, including Aren't We All? (1923), The Last of Mrs Cheyney (1925) and On Approval (1927) and the murder melodrama But for the Grace of God (1946). Some of his plays and musicals were made into films, and he also wrote a few screenplays.

Born and raised in Jersey, Lonsdale began writing comic sketches while serving in the army. His first play, Who's Hamilton?, was produced in 1903. In 1904 he eloped with Leslie Brooke Hoggan, through whom he was introduced to Frank Curzon. Curzon began to produce Lonsdale's musicals and comic plays in 1908 in the West End with The King of Cadonia. The Maid of the Mountains, opening in 1917, ran for 1,352 performances, becoming the second-longest-running musical in West End history. Through the 1920s, many of Lonsdale's musicals and plays enjoyed success. After this, his familiar genres, Edwardian musical comedy and drawing-room comedy, lost popularity. He continued to write plays and some screenplays for another two decades, moving to the United States in 1938.