An outboard propulsion system for vessels includes a supporting structure (4) to be anchored to the transom wall (5) of the vessel and a plurality of propellers (3) carried by the supporting structure (4). Each propeller (3) is associated to an electric motor (6) with a toroid geometry, having an annular rotor (R) rotatable within an annular stator (S) and defining therewithin a central aperture (A), with the blades (3A) of the propeller (3) which are carried by the rotor (R) and extend to the central aperture (A).