An outboard engine assembly for use on a boat has an engine mechanism including in an upper region thereof an engine having a substantially vertical crankshaft and a vertical array of substantially horizontal cylinders. The engine mechanism is mounted on the stern of the boat by an attachment. The attachment includes a stern bracket adapted to be fixed to the stern of the boat, a swivel case supported on the stern bracket for vertical angular movement, a substantially vertical swivel shaft supported on the swivel case for horizontal angular movement, and a pair of upper and lower supports mounted on the swivel shaft and supporting the engine mechanism. The engine has joint members coupled to the upper support at a position between adjacent two crankshaft counterbalances which correspond to adjacent two cylinders, respectively, as viewed in side elevation, the joint members having respective portions located within a path of the two crankshaft counterbalances as viewed in plan. The engine mechanism also has a balancer mechanism mounted on a front wall of the engine for attenuating engine vibrations, and a fuel supply assembly mounted on an engine side wall.