Sir Thomas Armstrong (c. 1633 – 20 June 1684) was an army officer and MP executed for Treason.
During the Interregnum he was a supporter of Charles II, participating in the plot to seize Chester Castle in 1655, and carrying funds from Aubrey de Vere, 20th Earl of Oxford to Charles in exile. He was possibly imprisoned for a year on his return. In 1657 he married Catherine niece of Edward Hyde, 1st Earl of Clarendon. A True Account of the Apprehending of Sir Thomas Armstrong, printed at London, 1684Following the Restoration, he received, in February 1661, a commission with the Horse Guards. In August 1675 Armstrong killed the son of one of the queen's ladies-in-waiting at a London theatre. Armstrong was pardoned on the grounds that his opponent had drawn first.
Armstrong served with James Scott, 1st Duke of Monmouth in France from 1672, fighting at the Siege of Maastricht (1673) and alongside the Dutch, in 1678. He was wounded at St Denis. In 1679 helped suppress the covenanter rising and fought at the battle at Bothwell Bridge.
Monmouth's influence secured him as MP for Stafford in March 1679 to the first Exclusion Parliament. Following the Rye House Plot in 1683 he was indicted for high treason. He fled to Clevesand then Rotterdam but was captured and sentenced to death by Judge Jeffreys. Armstrong was executed on 20 June 1684. His head was affixed to Westminster Hall, three of his quarters were displayed in London, and the fourth at Stafford.