Disclosed are a reservoir tank and an electronic brake system including the same. A reservoir tank according to an embodiment of the present disclosure includes an upper casing including an upper flange provided at a lower side thereof, and upper reinforcement ribs provided along an outer peripheral surface of the upper flange, and a lower casing including a lower flange provided at an upper side thereof, and lower reinforcement ribs provided along an outer peripheral surface of the lower flange, the lower casing being coupled to the lower side of the upper casing to form an internal space in which brake oil is stored, in which the upper reinforcement rib and the lower reinforcement rib are disposed on a straight line.