Sir Thomas Fairfax (17 January 1612 – 12 November 1671) was an English army officer and politician who commanded the New Model Army from 1645 to 1650 during the English Civil War. Because of his dark hair, he was known as "Black Tom" to his loyal troops. He was the eldest son and heir of Ferdinando Fairfax, 2nd Lord Fairfax of Cameron, (Lord Fairfax) and succeeded to that title as 3rd Lord Fairfax in 1648 on the death of his father, although he was generally known as "Sir Thomas Fairfax" to distinguish them. He adopted the profession of arms as a young man, when he served under Horace Vere in the Netherlands. In 1637, he married Vere's daughter Anne.
Fairfax was recalled to English service in 1639, for the first of King Charles' disastrous Bishops' Wars against Scotland. When these defeats led to the outbreak of the Civil War in 1642, Lord Fairfax declared for Parliament and was named general of Parliament's forces in the north, with Sir Thomas his second-in command. Sir Thomas later moved to join Parliament's stronger Eastern Association army, with which he achieved several significant victories, notably the decisive Battle of Marston Moor.