Sir John Fenwick, 3rd Baronet (c. 1645 – 28 January 1697) was an English Jacobite conspirator, who succeeded to the Baronetcy of Fenwick on the death of his father in 1676. He was the eldest son of Sir William Fenwick, or Fenwicke, a member of an old Northumberland family. He entered the army, becoming major-general in 1688, but before this date he had been returned in succession to his father as one of the Members of Parliament for Northumberland, which county he represented from 1677 to 1687. He was a strong partisan of King James II, and in 1685 was one of the principal supporters of the act of attainder against the Duke of Monmouth; but he remained in England when William III ascended the throne in the Revolution of 1688.