The present disclosure relates to a liquefied gas storage tank and a marine structure including the same. The storage tank includes a plurality of liquefied gas storage tanks received in a plurality of spaces defined in a hull of the marine structure by a cofferdam and arranged in two rows. The cofferdam includes at least one longitudinal cofferdam extending in a longitudinal direction of the hull and at least one transverse cofferdam extending in a transverse direction of the hull. Each of the storage tanks is sealed and thermally insulated by a sealing wall and a thermal insulation wall extending without being disconnected. The longitudinal cofferdam supports load of an upper structure while suppressing a sloshing phenomenon.