Thomas I may refer to:

• Thomas I of Constantinople, Patriarch from 607 to 610
• 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, Archbishop of Esztergom (1224)
• Thomas I of Saluzzo (died in 1296)
• Thomas I Komnenos Doukas (c. 1285–1318)
• Baselios Thomas I (1929– )

• Guaraci, Sao Paulo, Brazil
• Guaraci, São Paulo, Brazil
• Guaraculen
• Guaraculén
• Guaraldi
• Guaramacal salamander
• Guaramid dynasty
• Guaramids
• Guaram III of Iberia
• Guaram II of Iberia
• Guaram I of Iberia
• Guaramiranga
• Guaram Mampali
• Guarana Backus
• Guarana (energy drink)
• Guarana Jesus
• Guarana Knjaz Milos
• Guarana seed extract
• Guaranda, Sucre
• Guaranga
• Guarango
• Guarani
• Guarani alphabet
• Guarania (music)
• Guarani Antonio Franco