Self-priming centrifugal pump, comprising a pump body provided with an inner cavity having a longitudinal axis arranged in use in a horizontal position, said axis defining into said inner cavity an upper zone and a lower zone; a suction opening made on a frontal surface of said pump body at said upper zone, located above said longitudinal axis; an outlet opening made on an upper surface of said pump body; a pumping chamber associated with said pump body and having at a frontal part thereof an axial inlet opening; an impeller group arranged into said pumping chamber and provided with an axial hub configured to be associated with a motor shaft coaxial thereto, according to said longitudinal axis; an ejector group which is located into said inner cavity of the pump body and which is axially associated with said pumping chamber, said ejector group comprising an ejection nozzle and a collecting suction conduit shaping a diverging profile, connected, at the end of the wider section thereof, to said inlet opening of the pumping chamber, a feeding conduit configured for connecting said suction opening with said collecting suction conduit, at said ejection nozzle, an auxiliary suction conduit arranged into said inner cavity of the pump body, at said upper zone located above said horizontal axis, and configured for directly connecting said suction opening with said inlet opening of the pumping chamber, characterized in that it comprises a non-return valve arranged inside said auxiliary suction conduit for blocking the same auxiliary suction conduit when the pressure at the zone of the collecting suction conduit close to the mouth of said inlet opening of the pumping chamber is higher than the pressure at the upper part of the auxiliary suction conduit. a pump body provided with an inner cavity having a longitudinal axis arranged in use in a horizontal position, said axis defining into said inner cavity an upper zone and a lower zone; a suction opening made on a frontal surface of said pump body at said upper zone, located above said longitudinal axis; an outlet opening made on an upper surface of said pump body; a pumping chamber associated with said pump body and having at a frontal part thereof an axial inlet opening; an impeller group arranged into said pumping chamber and provided with an axial hub configured to be associated with a motor shaft coaxial thereto, according to said longitudinal axis; an ejector group which is located into said inner cavity of the pump body and which is axially associated with said pumping chamber, said ejector group comprising an ejection nozzle and a collecting suction conduit shaping a diverging profile, connected, at the end of the wider section thereof, to said inlet opening of the pumping chamber, a feeding conduit configured for connecting said suction opening with said collecting suction conduit, at said ejection nozzle, an auxiliary suction conduit arranged into said inner cavity of the pump body, at said upper zone located above said horizontal axis, and configured for directly connecting said suction opening with said inlet opening of the pumping chamber, characterized in that it comprises a non-return valve arranged inside said auxiliary suction conduit for blocking the same auxiliary suction conduit when the pressure at the zone of the collecting suction conduit close to the mouth of said inlet opening of the pumping chamber is higher than the pressure at the upper part of the auxiliary suction conduit.