A method for installing 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 at sea an empty basket of the counterweight of the floating support structure; descending into the sea the empty basket of the counterweight on which temporary floatation modules have been previously fixed and mooring it to a mooring previously placed at the seabed; activating the temporary floatation modules of the basket to allow it to be stabilized in mid-water vertically to the mooring; towing at sea the float of the floating support structure up to the vertical of the empty basket of the counterweight; connecting the main structure of the float to the empty basket of the counterweight by means of ballast links; sinking the empty basket of the counterweight; disconnecting the mooring and the temporary floatation modules of the basket of the counterweight; filling the basket of the counterweight with ballast material in order to partially immerse the float; and towing at sea the float connected to the counterweight up to the area of implantation of the wind turbine. 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 at sea an empty basket of the counterweight of the floating support structure; descending into the sea the empty basket of the counterweight on which temporary floatation modules have been previously fixed and mooring it to a mooring previously placed at the seabed; activating the temporary floatation modules of the basket to allow it to be stabilized in mid-water vertically to the mooring; towing at sea the float of the floating support structure up to the vertical of the empty basket of the counterweight; connecting the main structure of the float to the empty basket of the counterweight by means of ballast links; sinking the empty basket of the counterweight; disconnecting the mooring and the temporary floatation modules of the basket of the counterweight; filling the basket of the counterweight with ballast material in order to partially immerse the float; and towing at sea the float connected to the counterweight up to the area of implantation of the wind turbine. transporting at sea an empty basket of the counterweight of the floating support structure; descending into the sea the empty basket of the counterweight on which temporary floatation modules have been previously fixed and mooring it to a mooring previously placed at the seabed; activating the temporary floatation modules of the basket to allow it to be stabilized in mid-water vertically to the mooring; towing at sea the float of the floating support structure up to the vertical of the empty basket of the counterweight; connecting the main structure of the float to the empty basket of the counterweight by means of ballast links; sinking the empty basket of the counterweight; disconnecting the mooring and the temporary floatation modules of the basket of the counterweight; filling the basket of the counterweight with ballast material in order to partially immerse the float; and towing at sea the float connected to the counterweight up to the area of implantation of the wind turbine.