Fantasia on a Theme by Thomas Tallis, also known as the Tallis Fantasia, is a one-movement work for string orchestra by Ralph Vaughan Williams. The theme is by the 16th-century English composer Thomas Tallis. The Fantasia was first performed at Gloucester Cathedral as part of the 1910 Three Choirs Festival, and has entered the orchestral repertoire, with frequent concert performances and recordings by conductors and orchestras of various countries.