Thomas Rainsborough (1610 – 30 October 1648), or Rainborough or Raineborough or Rainborowe or Rainbow or Rainborow, was a prominent figure in the English Civil War, and was the leading spokesman for the Ranters in the Putney Debates.
He was the son of William Rainsborough, a captain and Vice-Admiral in the Royal Navy, and Ambassador to Morocco (for his services to end white slavery he was offered a baronetcy, which he declined). Before the war, Thomas and his brother, William Rainsborowe, were both involved in an expedition to the Puritan Providence Island colony, off the coast of Nicaragua. Rainsborough commanded the Swallow and other English naval vessels in the first civil war. By May 1645, he was a colonel in the New Model Army, taking an active part in the battles at Naseby and at Bristol. Later that year, he captured the symbolic stronghold of Berkeley Castle. In 1646, he helped conclude the Siege of Worcester.
In January 1647, Rainsborough became a member of parliament for Droitwich. He was the highest ranking supporter of the Ranters in the New Model Army and one of the speakers for the Ranter side in the Putney Debates (July 1647), where he opposed any deal with the King.
In early 1648, he was due to return to the Navy as a Vice-Admiral, but his Leveller sympathies were unpopular with some officers, and a mutiny ensued. He was returned to Army service.
In October, Rainsborough was sent by his commander, Sir Thomas Fairfax, to the siege at Pontefract Castle, where he was killed by four Royalists during a bungled kidnap attempt. His funeral was the occasion for a large Leveller-led demonstration in London, with thousands of mourners wearing the Levellers' ribbons of sea-green and bunches of rosemary for remembrance in their hats. After his death, his brother, William Rainsborowe continued in the Ranter) cause.