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.

A string orchestra is an orchestra consisting solely of a string section made up of the bowed strings used in Western Classical music. The instruments of such an orchestra are most often the following: the violin, which is divided into first and second violin players, the viola, the cello, and usually, but not always, the double bass.