A control device for a marine engine comprised so that a section of an angle sufficiently narrower than a crank angle corresponding to each stroke of a combustion cycle of the engine is set as an instantaneous speed detection section, a crank angle position that appears every time the engine rotates in the instantaneous speed detection section is detected as a specific crank angle position, an instantaneous rotational speed of the engine is detected every time each specific crank angle position is detected, it is determined whether the engine needs to be assisted from the degree of reduction in the instantaneous rotational speed, and a motor-generator including a rotor directly connected to a crankshaft of the engine is driven to apply a drive force from the motor-generator to the engine when it is determined that the engine needs to be assisted.