An implantable prosthesis for percutaneous placement within a vein that forces opposing portions of the vessel wall of a vein together to create a new valve of autologous vein tissue to be operable to alternate between a valve closed configuration and a valve open configuration. When in a preset closed configuration, the implantable prosthesis pushes or pulls portions of the vessel wall of the vein together to substantially close the vein lumen and prevent retrograde blood flow from backflowing through the new valve in the valve closed configuration. The implantable prosthesis has leg portions that may be pushed apart in response to antegrade blood flow through the vein to allow the new valve to achieve the valve open configuration.