A method for installing at sea an offshore wind turbine provided with a floating support structure which comprises a float intended to be partly immersed and on which a wind turbine mast is intended to be assembled, and a counterweight connected to the float and intended to be immersed under the float, the float comprising: a toroid or polygon-shaped main structure with at least five sides which is formed by at least one tube intended to be immersed; a central tubular structure having a diameter adapted to receive the mast of the wind turbine and comprising a section able to be ballasted in order to adjust the waterline of the float; a first series of horizontal struts evenly distributed about a vertical axis and connecting the main structure to the central structure; and a second series of oblique struts evenly distributed about a vertical axis (Y-Y) and connecting the main structure to the central structure at an angle ( α) comprised between 15° and 60° with the horizontal struts; the counterweight comprising a basket able to receive ballast material and ballast links connecting the basket to the main structure of the float at an angle ( β) comprised between 15° and 45° with the vertical axis (Y-Y), the method comprising the successive steps of: transporting jointly a basket of the counterweight positioned under the floating support structure and its float; vertically descending the basket by means of a lifting system integrated at the level of the float; towing at sea the float connected to the counterweight up to the area of implantation of the wind turbine; and filling with ballast material the basket of the counterweight suspended from the structure of the float. a toroid or polygon-shaped main structure with at least five sides which is formed by at least one tube intended to be immersed; a central tubular structure having a diameter adapted to receive the mast of the wind turbine and comprising a section able to be ballasted in order to adjust the waterline of the float; a first series of horizontal struts evenly distributed about a vertical axis and connecting the main structure to the central structure; and a second series of oblique struts evenly distributed about a vertical axis (Y-Y) and connecting the main structure to the central structure at an angle ( α) comprised between 15° and 60° with the horizontal struts; the counterweight comprising a basket able to receive ballast material and ballast links connecting the basket to the main structure of the float at an angle ( β) comprised between 15° and 45° with the vertical axis (Y-Y), the method comprising the successive steps of: transporting jointly a basket of the counterweight positioned under the floating support structure and its float; vertically descending the basket by means of a lifting system integrated at the level of the float; towing at sea the float connected to the counterweight up to the area of implantation of the wind turbine; and filling with ballast material the basket of the counterweight suspended from the structure of the float. transporting jointly a basket of the counterweight positioned under the floating support structure and its float; vertically descending the basket by means of a lifting system integrated at the level of the float; towing at sea the float connected to the counterweight up to the area of implantation of the wind turbine; and filling with ballast material the basket of the counterweight suspended from the structure of the float.