An oral irrigator with back flow prevention is described. The oral irrigator may include a reservoir, a base, a handle, and a one-way or unidirectional valve. The base may support the reservoir and include a pump fluidly connected to the reservoir. The handle may be positioned external the base and in fluid communication with the pump via a hose. The valve may be positioned external the base to inhibit or seal fluid from flowing from a downstream side of the valve toward the pump.