An apparatus for treating a substrate includes a process chamber having a treatment space defined therein, a support unit for supporting the substrate in the treatment space, a fluid supply unit for supplying supercritical fluid to the treatment space, and a controller configured to control the fluid supply unit, wherein the fluid supply unit is configured to selectively supply the supercritical fluid at a first density or a second density higher than the first density into the treatment space. Thus, drying efficiency of the substrate when drying the substrate using the supercritical fluid may be improved.