Method for installing an offshore wind turbine at a target location at sea with an installation vessel, wherein the installation vessel floats during the installation of the wind turbine, the installation vessel comprising: - a nacelle support structure for temporarily supporting a nacelle comprising a hub having a plurality of root end connectors to which root ends of the blades are to be connected, the nacelle support structure comprising: o a support tower extending upwardly from a deck of the installation vessel, wherein a base of the support tower remains at a fixed position relative to the deck during the installation of the wind turbine, o a support platform connected to the support tower, the support platform being configured to temporarily support the nacelle during the connecting of one or more blades thereto, - one or more lifting devices (34,35) configured for: o lifting the nacelle onto the support platform, o lifting a nacelle assembly including the blades onto a wind turbine mast located adjacent the vessel, wherein the lifting device for lifting the completed nacelle assembly is a crane which is separate from the support tower, wherein the method comprises: a) lifting the nacelle onto the support platform, and securing the nacelle to the support platform, b) orienting a root end connector of the hub of the nacelle in a direction facing a guide path of a blade moving system, c) connecting the root end of the first blade to the corresponding first root end connector of the hub, d) repeating steps b) and c) for subsequent blades and root end connectors until all blades are connected to the hub of the nacelle, thereby providing a Rotor Nacelle Assembly, also referred to as RNA, e) suspending the RNA from the crane, lifting the RNA from the support platform of the nacelle support structure with the crane, subsequently moving the RNA with the crane away from the support platform and positioning the RNA onto a wind turbine mast located adjacent the vessel. - a nacelle support structure for temporarily supporting a nacelle comprising a hub having a plurality of root end connectors to which root ends of the blades are to be connected, the nacelle support structure comprising: o a support tower extending upwardly from a deck of the installation vessel, wherein a base of the support tower remains at a fixed position relative to the deck during the installation of the wind turbine, o a support platform connected to the support tower, the support platform being configured to temporarily support the nacelle during the connecting of one or more blades thereto, o a support tower extending upwardly from a deck of the installation vessel, wherein a base of the support tower remains at a fixed position relative to the deck during the installation of the wind turbine, o a support platform connected to the support tower, the support platform being configured to temporarily support the nacelle during the connecting of one or more blades thereto, - one or more lifting devices (34,35) configured for: o lifting the nacelle onto the support platform, o lifting a nacelle assembly including the blades onto a wind turbine mast located adjacent the vessel, wherein the lifting device for lifting the completed nacelle assembly is a crane which is separate from the support tower, o lifting the nacelle onto the support platform, o lifting a nacelle assembly including the blades onto a wind turbine mast located adjacent the vessel, wherein the lifting device for lifting the completed nacelle assembly is a crane which is separate from the support tower, a) lifting the nacelle onto the support platform, and securing the nacelle to the support platform, b) orienting a root end connector of the hub of the nacelle in a direction facing a guide path of a blade moving system, c) connecting the root end of the first blade to the corresponding first root end connector of the hub, d) repeating steps b) and c) for subsequent blades and root end connectors until all blades are connected to the hub of the nacelle, thereby providing a Rotor Nacelle Assembly, also referred to as RNA, e) suspending the RNA from the crane, lifting the RNA from the support platform of the nacelle support structure with the crane, subsequently moving the RNA with the crane away from the support platform and positioning the RNA onto a wind turbine mast located adjacent the vessel.