Provided are an apparatus and a method for supplying a liquid. An apparatus for treating a substrate includes a substrate support unit to support the substrate, and a liquid supplying unit to supply a liquid to a substrate supported by the substrate support unit. The liquid supplying unit includes a nozzle to discharge the liquid, a liquid supplying line to supply the liquid to the nozzle, a constant pressure valve installed on the liquid supplying line, a shut-off valve installed on the liquid supplying line while being interposed between the constant pressure valve and the nozzle to supply the liquid or to stop the supply of the liquid, and a controller to control the constant pressure valve and the shut-off valve.