A bicycle hydraulic operating device is basically provided with a base member, a piston, an operating member, a hydraulic reservoir tank and a diaphragm. The base member includes a cylinder bore. The piston is movably disposed in the cylinder bore. The operating member is coupled to the piston to move the piston within the cylinder bore. The hydraulic reservoir tank is fluidly connected to the cylinder bore. The diaphragm is at least partially disposed inside the hydraulic reservoir tank and divides the hydraulic reservoir tank into a hydraulic fluid chamber and an air chamber. The air chamber is at least partially located closer to the cylinder bore than the hydraulic fluid chamber.