Sir Thomas Cullinan (12 February 1862 – 23 August 1936) was a South African diamond magnate. He is renowned for giving his name to the Cullinan Diamond, the largest diamond ever discovered, and as owner of the Premier Mine, now renamed the Cullinan Mine, from which the famous gem was extracted on 26 January 1905. He also gave his name to the nearby South African town of Cullinan.

Thomas Cullinan was born in Elands Post near Seymore, Cape Colony on 12 February 1862.