HMS Isis was a 50-gun Portland-class fourth-rate ship of the Royal Navy.

She was built in 1774 on the River Medway and commissioned under Captain Charles Douglas in 1776, at which time he sailed with a squadron for the relief of Quebec.

The ship was also engaged in the action of 22 August 1795, off Norway, against a Dutch squadron.

She then served as the flagship of Vice-Admiral Sir Andrew Mitchell during the 1799 Anglo-Russian invasion of Holland.

[1] She was badly damaged by a hurricane during the Peace of Amiens on crossing the Atlantic to be Vice Admiral Gambier's flagship in Newfoundland, before going on to further service in Newfoundland, the Caribbean, and the North Sea.