Sir John in Love is an opera in four acts by the English composer Ralph Vaughan Williams. The libretto, by the composer himself, is based on Shakespeare's The Merry Wives of Windsor and supplemented with texts by Philip Sidney, Thomas Middleton, Ben Jonson, and Beaumont and Fletcher. The music deploys English folk tunes, including "Greensleeves". Originally titled The Fat Knight, the opera premiered at the Parry Opera Theatre, Royal College of Music, London, on 21 March 1929. Its first professional performance was on 9 April 1946 at Sadler's Wells Theatre.