HMS Union was a 90-gun second rate ship of the line of the Royal Navy, built at Chatham Dockyard to the draught specified by the 1745 Establishment as amended in 1750, and launched on 25 September 1756. In 1756, one of the midshipmen on the Union was John Hunter, later to become an admiral and the second Governor of New South Wales. In 1799, Union was converted to serve as a hospital ship, and continued in this role until 1816, when the decision was taken to have her broken up.