Sir Thomas Gresham the Elder (/ˈɡrɛʃəm/; c. 1519 – 21 November 1579) was an English merchant and financier who acted on behalf of King Edward VI (1547–1553) and Edward's half-sisters, queens Mary I (1553–1558) and Elizabeth I (1558–1603). In 1565 Gresham founded the Royal Exchange in the City of London.

Born in London and descended from an old Norfolk family, Gresham was one of two sons and two daughters of Sir Richard Gresham, a leading merchant mercer and Lord Mayor of London, who was knighted by King Henry VIII for negotiating favourable loans with foreign merchants.

Gresham was educated at St Paul's School. After that, although his father wanted Thomas to become a merchant, Sir Richard first sent him to university at Gonville and Caius College, Cambridge. He was concurrently apprenticed in the Mercers' Company to his uncle Sir John Gresham, founder of Gresham's School, while he was still at Cambridge.

In 1543 the Mercers' Company admitted the 24-year-old Gresham as a liveryman, and later that year he left England for the Low Countries, where, either on his own account or that of his father or uncle, he carried on business as a merchant whilst acting in various matters as agent for King Henry VIII. In 1544 he married Anne Ferneley, widow of the London merchant Sir William Read, but maintained residence principally in the Low Countries, basing his headquarters at Antwerp in present-day Belgium (then the Spanish Netherlands), where he became renowned for his adept market-play.

When in 1551 the mismanagement of Sir William Damsell, King's Merchant to the Low Countries, had caused the English Government much financial embarrassment, the authorities called Gresham for advice, thereafter following his proposals. Gresham advocated the adoption of various methods – highly ingenious, but quite arbitrary and unfair – for raising the value of the pound sterling on the Stock Exchange in Antwerp which proved so successful that in just a few years King Edward VI had discharged almost all of his debts. The Government sought Gresham's advice in all their money difficulties, and also frequently employed him in various diplomatic missions. He had no stated salary, but in reward of his services received from King Edward various grants of lands, the annual value of which at that time amounted ultimately to about 400 pounds a year.

On the accession of Queen Mary in 1553, Gresham fell out of favour at Court for a short time with Alderman William Dauntsey displacing him. But Dauntsey's financial operations proved unsuccessful and Gresham was soon reinstated; and as he professed his zealous desire to serve the Queen, and manifested great adroitness both in negotiating loans and in smuggling money, arms and foreign goods, not only were his services retained throughout her reign (1553–1558), but besides his salary of twenty shillings per diem he received grants of church lands to the yearly value of 200 pounds.

Under Queen Elizabeth's reign (1558–1603), besides continuing in his post as financial agent of the Crown, Gresham acted as Ambassador Plenipotentiary to the Court of Duchess Margaret of Parma, Governor of the Netherlands, and was appointed a Knight Bachelor in 1559 prior to his departure. The unsettled times preceding the Dutch revolt compelled him to leave Antwerp on 10 March 1567; but, though he spent the remainder of his life in London, he continued his business as merchant and government financial agent in much the same way as he had always done.

Queen Elizabeth also found Gresham's abilities useful in a variety of other ways, including acting as gaoler to Lady Mary Grey (sister of Lady Jane Grey), who, as a punishment for marrying Thomas Keyes the sergeant-porter, was imprisoned both in his Bishopsgate house as well as his other home, Osterley Park, from June 1569 to the end of 1572. The second home he sometimes used to entertain the Queen. Grey was related to Gresham's wife, Anne, via the Wingfield family in Suffolk.