Plant of production of electric power from the movement of waves comprising: an offshore dam for separating an inside section of sea in a port zone, from an open-sea section, a submerged part which has one or more ducts (14, 15) for receiving the water arranged towards said open-sea section, non-return valve means (8, 10, 29) provided upstream and/or downstream said one or more ducts (14, 15) at least one turbine connected to electric power generating means being positioned in said offshore dam or in a land zone also at a distance from said offshore dam, said turbine being connected upstream to at least one water supply duct (11, 25) in turn connected upstream to suitable pumping means able to convey the water received via these ducts (14, 15) towards said duct (11, 25) for supplying water to said turbine, said hydraulic pump being provided with at least one cylinder inside which a piston connected at the top to a rod which protrudes from said cylinder slides vertically, said rod being connected to at least one float (2, 2') able to move upwards and downwards following the oscillating movement of the sea waves, said cylinder is provided, on the side of the open-sea section, with at least one first duct for receiving the water, situated close to the top thereof, and a second duct for receiving the water, situated close to the base thereof, characterized in said cylinder being provided, on the side of the offshore dam, with at least one third duct situated close to the top thereof and with a fourth duct situated close to the base thereof, said third and fourth ducts (12, 13) being connected to said duct for supplying the water to the turbine. an offshore dam for separating an inside section of sea in a port zone, from an open-sea section, a submerged part which has one or more ducts (14, 15) for receiving the water arranged towards said open-sea section, non-return valve means (8, 10, 29) provided upstream and/or downstream said one or more ducts (14, 15) at least one turbine connected to electric power generating means being positioned in said offshore dam or in a land zone also at a distance from said offshore dam, said turbine being connected upstream to at least one water supply duct (11, 25) in turn connected upstream to suitable pumping means able to convey the water received via these ducts (14, 15) towards said duct (11, 25) for supplying water to said turbine, said hydraulic pump being provided with at least one cylinder inside which a piston connected at the top to a rod which protrudes from said cylinder slides vertically, said rod being connected to at least one float (2, 2') able to move upwards and downwards following the oscillating movement of the sea waves, said cylinder is provided, on the side of the open-sea section, with at least one first duct for receiving the water, situated close to the top thereof, and a second duct for receiving the water, situated close to the base thereof, characterized in said cylinder being provided, on the side of the offshore dam, with at least one third duct situated close to the top thereof and with a fourth duct situated close to the base thereof, said third and fourth ducts (12, 13) being connected to said duct for supplying the water to the turbine.