A steering system for a marine vessel having a propulsion unit, the steering system comprising: a propulsion unit including a tilt tube, a support rod received by the tilt tube and a tiller; and an electric actuator for imparting steering movement to the propulsion unit, the electric actuator including: a housing; an output shaft (24a, 24b) reciprocatingly received by the housing; a screw assembly disposed within the housing and coupled to the output shaft (24a, 24b), the screw assembly including a plurality of annular rollers and a central screw received by the annular rollers, the annular rollers being rotatable about the central screw; a motor including a stator and a rotor, the rotor having an inner bore which engages the annular rollers, wherein rotation of the rotor causes the central screw to translate axially relative to the rotor and the output shaft (24a, 24b) to reciprocate relative to the housing; a pivot plate pivotably connected to the tiller, the pivot plate rotationally constraining the housing of the electric actuator for providing a reaction torque for rotation of the rotor; and support arms (40, 42) which connect respective ends of the output shaft (24a, 24b) to the support rod of the propulsion unit, the support arms (40, 42) inhibiting axial movement of the output shaft (24a, 24b) relative to the marine vessel while the housing of the electric actuator reciprocates along the output shaft (24a, 24b) and linearly relative to the marine vessel. a propulsion unit including a tilt tube, a support rod received by the tilt tube and a tiller; and an electric actuator for imparting steering movement to the propulsion unit, the electric actuator including: a housing; an output shaft (24a, 24b) reciprocatingly received by the housing; a screw assembly disposed within the housing and coupled to the output shaft (24a, 24b), the screw assembly including a plurality of annular rollers and a central screw received by the annular rollers, the annular rollers being rotatable about the central screw; a motor including a stator and a rotor, the rotor having an inner bore which engages the annular rollers, wherein rotation of the rotor causes the central screw to translate axially relative to the rotor and the output shaft (24a, 24b) to reciprocate relative to the housing; a pivot plate pivotably connected to the tiller, the pivot plate rotationally constraining the housing of the electric actuator for providing a reaction torque for rotation of the rotor; and support arms (40, 42) which connect respective ends of the output shaft (24a, 24b) to the support rod of the propulsion unit, the support arms (40, 42) inhibiting axial movement of the output shaft (24a, 24b) relative to the marine vessel while the housing of the electric actuator reciprocates along the output shaft (24a, 24b) and linearly relative to the marine vessel. a housing; an output shaft (24a, 24b) reciprocatingly received by the housing; a screw assembly disposed within the housing and coupled to the output shaft (24a, 24b), the screw assembly including a plurality of annular rollers and a central screw received by the annular rollers, the annular rollers being rotatable about the central screw; a motor including a stator and a rotor, the rotor having an inner bore which engages the annular rollers, wherein rotation of the rotor causes the central screw to translate axially relative to the rotor and the output shaft (24a, 24b) to reciprocate relative to the housing; a pivot plate pivotably connected to the tiller, the pivot plate rotationally constraining the housing of the electric actuator for providing a reaction torque for rotation of the rotor; and support arms (40, 42) which connect respective ends of the output shaft (24a, 24b) to the support rod of the propulsion unit, the support arms (40, 42) inhibiting axial movement of the output shaft (24a, 24b) relative to the marine vessel while the housing of the electric actuator reciprocates along the output shaft (24a, 24b) and linearly relative to the marine vessel.