Sir James Montagu SL KC (2 February 1666 – 1723) was an English barrister, and judge. As a politician, he sat in the House of Commons at various times between 1695 and 1713 and served as Solicitor General and Attorney General.
Montagu was the sixth son of George Montagu of Horton, Northamptonshire, by his wife Elizabeth Irby, daughter of Sir Anthony Irby. His grandfather was Henry Montagu, 1st Earl of Manchester. He was entered at the Middle Temple, and called to the bar.
In 1695 Montagu became Member of Parliament for Tregony, and for Beeralston in 1698, when he was also made chief justice of Ely. In 1704 he successfully defended John Tutchin, indicted for a libel published in his periodical, The Observator, and two years later he was leading counsel in the prosecution of Beau Fielding for bigamy in marrying the Duchess of Cleveland. In 1705 he was committed by the House of Commons to the custody of the serjeant-at-arms for having in 1704 demanded a habeas corpus on behalf of the group of Aylesbury men, whom the house had committed to Newgate Prison for bringing actions against the returning officer; Montagu pleaded strongly against the privilege claimed by the Commons. He remained in custody from 26 February to 14 March, when parliament was prorogued and afterwards dissolved. In April 1705 he was knighted at Cambridge, and was made a Q.C. in November of the same year.