The San Carlos River (Spanish: Rio San Carlos) is a river in the Alajuela Province of Costa Rica that drains into the San Juan River and thus into the Caribbean Sea. Its basin covers an area of about 3,100 km2. The river has a length of 142 km of which about 60 km allow navigation. The river originates from the confluence of the Jabillos River and Peje River both originating on the eastern side of the Cordillera de Tilarán and traverses the San Carlos Canton.