Robert Montagu, 3rd Earl of Manchester JP (baptised 25 April 1634 – 14 March 1683) was an English courtier and politician who sat in the House of Commons from 1660 to 1671 when he inherited the peerage as Earl of Manchester.
Montagu was the son of Edward Montagu, 2nd Earl of Manchester and his second wife Lady Anne Rich, daughter of Robert Rich, 2nd Earl of Warwick. He was born in the parish of St. Margaret's, Westminster, and baptised there on 25 April 1634. From 1649 to 1654, he was travelling abroad. He was captain of the militia horse for Huntingdonshire in April 1660.
Also in April 1660, Montagu was returned as Member of Parliament for Huntingdonshire in the Convention Parliament and in the following month was one of the members who waited on the king at the Hague. He was a J.P. for Huntingdonshire and Northamptonshire, from July 1660 to 1681. Also in July, he was commissioner for oyer and terminer for the Midland circuit. From August 1660, he was commissioner for assessment and Deputy Lieutenant for Huntingdonshire until 1671.
In 1661, Montagu was again elected MP for Huntingdonshire in the Cavalier Parliament. In 1663, he became commissioner for assessment for Huntingdon and Northamptonshire until 1671, and was sent on a mission to the French king. He was created M.A. by the university of Oxford, on 8 September 1665. In February 1666, he succeeded the Earl of Newport as gentleman of the bedchamber to the king. In 1666 and 1667 Montagu was a captain of the Duke of Monmouth’s Horse in the eastern counties while the Dutch were on the coast.
Montagu inherited the title of Earl of Manchester on the death of his father in 1671 and took on the posts of Lord Lieutenant of Huntingdonshire and Custos Rotulorum In 1672 he became master of the swans and also water-bailiff for Whittlesey Mere. He became high steward of Cambridge University in 1677.