A fluid supply instrument for a treatment instrument that is mounted on a grasping section of the treatment instrument capable of grasping a living tissue includes a port configured to receive a fluid, and a cover continuous to the port and configured to be covering a back surface on an opposite side of a grasping surface of the grasping section that is capable of grasping a living tissue. The cover forms a fluid channel continuous from the port in cooperation with the grasping section, and allows the fluid received from the port to flow out from between a cover edge adjacent to the grasping surface and an edge portion of the grasping surface.