Disclosed are a water-expandable rubber composition and a water-expandable rubber pad including the same. The water-expandable rubber pad manufactured using the water-expandable rubber composition is rigid enough to be directly inserted into a cylinder jacket even without a metal plate, etc. Moreover, the water-expandable rubber pad may increase in volume upon contact with engine coolant, thereby being optimized for an assembly process and facilitating design. Thus, since no separate assembly parts are used compared to conventional cases, the number of parts can be reduced, and costs can be reduced, and adhesiveness of the rubber pad is superior to that of conventional foam rubber and thus heat retention performance is excellent, resulting in improved engine fuel efficiency.