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 separately an empty and floating basket of the counterweight of the floating support structure and its float; connecting ballast links and sinking chains between the basket and the float; immersing the basket by gradually ballasting it and controlling its position by the catenary effect of the sinking chains to descend it into an equilibrium position under the float; descending the basket under the float until the ballast links are stretched; 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 at sea. 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 separately an empty and floating basket of the counterweight of the floating support structure and its float; connecting ballast links and sinking chains between the basket and the float; immersing the basket by gradually ballasting it and controlling its position by the catenary effect of the sinking chains to descend it into an equilibrium position under the float; descending the basket under the float until the ballast links are stretched; 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 at sea. transporting at sea separately an empty and floating basket of the counterweight of the floating support structure and its float; connecting ballast links and sinking chains between the basket and the float; immersing the basket by gradually ballasting it and controlling its position by the catenary effect of the sinking chains to descend it into an equilibrium position under the float; descending the basket under the float until the ballast links are stretched; 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 at sea.