John Forbes (17 July 1714 – 10 March 1796), styled The Honourable from 1734, was an officer of the Royal Navy who eventually rose to the rank of Admiral of the Fleet.
Forbes was born at Minorca as the second son of George Forbes, third earl of Granard and Mary, née Stewart (1672/3–1758), who was the eldest daughter of William, first Viscount Mountjoy. John followed his father into the Navy, joining the 70-gun third rate HMS Burford on 31 May 1726, at the age of 12. He served as a volunteer under the command of his maternal uncle, the Honourable Charles Stewart. He served with the Burford in the Mediterranean, before moving with his uncle to HMS Lion, and the two sailed together to the West Indies in 1729. Whilst serving here, Stewart promoted Forbes to lieutenant.
His first commission was as third lieutenant to the 60-gun HMS Kingston, which he joined on 13 January 1733. He was then appointed the following year to the post of fourth lieutenant on the larger 70-gun HMS Edinburgh. On 21 July he joined the 100-gun first rate HMS Britannia as sixth lieutenant, rising to third lieutenant by May 1735. The Britannia was at this time the flagship of Sir John Norris, and Forbes sailed with him on an expedition to Lisbon to support the Portuguese in the face of a Spanish threat.