The present disclosure relates to an apparatus and a method for treating a substrate with a liquid. A substrate treating apparatus includes a substrate supporting unit having a supporting plate for supporting a substrate and a bottom liquid supply unit for supplying a liquid to a bottom of the substrate supported by the supporting plate, wherein the bottom liquid supply unit includes a body and a liquid discharge nozzle for discharging a treatment liquid to the bottom of the substrate and coupled to the body and wherein an upper surface of the body includes a drainage hole for draining a liquid remaining in the body. Accordingly a liquid remained in the body may be discharged through the drainage hole.