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.
Eight ships of the Royal Navy have borne the name HMS Union:
HMS Albemarle was a 90-gun second rate ship of the line of the Royal Navy, launched in 1680 at Harwich.
She was rebuilt in 1704 at Chatham Dockyard, remaining a 90-gun second rater. She was also renamed HMS Union at this time. She underwent a second rebuild at Chatham, from where she was relaunched on 8 February 1726 as a 90-gun second rater built to the 1719 Establishment.
Union was broken up in 1749.
HMS Union was a 98-gun second rate ship of the line of the Royal Navy, launched on 16 November 1811 at Plymouth.
She was broken up in 1833.