Embodiments of the inventive concepts provide an apparatus and a method for treating a substrate. The apparatus includes a substrate support unit supporting the substrate, and a liquid supply unit supplying a treatment solution to the substrate supported by the substrate support unit. The liquid supply unit includes a nozzle discharging the treatment solution to the substrate, a liquid supply line supplying the treatment solution to the nozzle, a mixing member installed on the liquid supply line, a mixing space formed within the mixing member, and a gas supply line connected to the mixing member to supply a gas into the mixing space. The mixing member includes a body within which the mixing space is formed, and an inflow port combined with the body such that the gas supplied through the inflow port is mixed with the treatment solution in the mixing space to form nano-sized bubbles.