Gardiner, city, Kennebec county, southwestern Maine, U.S., on the Kennebec River (head of navigation) just south of Augusta and bounding the towns of Farmingdale, West Gardiner, and Richmond. Founded in 1754 by Sylvester Gardiner as Gardinerstown Plantation, it was set off from Pittston in 1760 and was incorporated as a town in 1803.