Thomas I may refer to:
Thomas I of Constantinople, Patriarch from 607 to 610 Thomas I of Jerusalem, Patriarch until 821 Thomas I of York (died in 1100) Thomas I, Count of Savoy (1178–1233) Thomas I d'Autremencourt (died ca. 1212), Lord of Salona Thomas I of Esztergom (1224) Thomas I of Saluzzo (died in 1296) Thomas I Komnenos Doukas (c. 1285–1318) Baselios Thomas I (1929– )