Gerasim Grigor'evich Izmailov (Герасим Григорьевич Измайлов) (circa 1745 - after 1795) was a Russian sea explorer who did much to map the Russian Far East and the Aleutian Islands and Russian America (Alaska).

Born in Yakutsk, he made numerous sea voyages together with Dmitry Bocharov , both of whom attended navigation school together. In 1771 both were caught up in the Benevsky mutiny on Bol'sheretsky island in Kamchatka. After being abandoned for more than a year in Robinson Crusoe fashion on uninhabited Simushir , one of the Kuril Islands, Izmailov managed to return to the mainland where he was detained by the authorities in Irkutsk until he was cleared in 1774.

From 1775–81 he led expeditions to the Aleutian Islands as commander of the St. Paul. In October 1778 he encountered Captain James Cook at Unalaska and presented him with a letter of introduction to the Russian authorities; this letter was used by Cook's successor Captain Charles Clerke after Cook died. Cook meanwhile handed a letter to Izmailov for delivery to London.