Sir Sidney Montagu (died 25 February 1644) was an English politician who sat in the House of Commons at various times between 1593 and 1642. He supported the Royalist cause in the English Civil War.
Montagu was the son of the judge Sir Edward Montagu, of Boughton, Northamptonshire and the grandson of another judge Sir Edward Montagu. He matriculated at Christ's College, Cambridge in December 1588 and was admitted at Middle Temple on 11 May 1593.
In 1593, Montagu was elected Member of Parliament for Brackley. He was elected MP for Malmesbury in 1601 and for Wells in 1614. He became Master of Requests to King Charles I and was Knighted on 28 July 1616.
In November 1640, Montagu was elected MP for Huntingdonshire in the Long Parliament. He was expelled and committed to the Tower of London in 1642.
Montagu was of Hinchingbrooke House, Huntingdonshire, England. He married Paulina Pepys, daughter of John Pepys, of Cottenham, Cambridgeshire, England and sister of Richard Pepys and Thomas Pepys, grandfather of Samuel Pepys. His son Edward was created Earl of Sandwich. His brothers included Edward Montagu, 1st Baron Montagu, Sir Walter Montagu, Henry Montagu, 1st Earl of Manchester, Sir Charles Montagu and James Montagu, Bishop of Winchester. Samuel Pepys, as he admitted, owed his start in life to the " chance without merit" which made his great-aunt the mother of the Earl of Sandwich.