Frederick Thesiger (28 March 1758 – 26 August 1805) was an English naval officer who went to sea with the East India Company and was later commissioned into the Royal Navy and the navy of the Russian Empire.
Thesiger saw active service in the American War of Independence, the Russo-Swedish War of 1788–1790, and the War of the Second Coalition. He was flag lieutenant to Rodney at the Battle of the Saintes and to Nelson at the Battle of Copenhagen and served onshore during the Napoleonic Wars.