Thomas the Tank Engine is an anthropomorphised fictional tank locomotive in the British Railway Series books by Wilbert Awdry and his son, Christopher, published from 1945. He became the most popular and famous character in the series, and is the titular protagonist in the accompanying television adaptation series Thomas & Friendsand its reboot Thomas & Friends: All Engines Go.

Thomas is a blue steam engine and has a number 1 painted on his side. All of the vehicles in The Railway Series were based on prototypical engines; Thomas's basis is the LB&SCR E2 class.Thomas first appeared in 1946 in the second book in the series, Thomas the Tank Engine, and was the focus of the four short stories contained within. In The Railway Series and early episodes of Thomas & Friends, Thomas's best friends are Percy and Toby, though he is also close friends with Edward. Later episodes of Thomas & Friends have Thomas in a trio with James and Percy, and Percy is known as his best friend.

In 1979, British writer and producer Britt Allcroft came across the books,and arranged a deal to bring the stories to life as the TV series Thomas the Tank Engine & Friends (later simplified to Thomas & Friends). The programme became an award-winning hit around the world, with a vast range of spin-off commercial products.