The San Carlos River is a river of Costa Rica, belonging to the northern sub-slope of the Caribbean slope of this country. It runs through the canton of the same name in the province of Alajuela. The San Carlos river basin covers an area of approximately 3100 km². The river crosses most of the canton's territory with a direction from south to north and its length is 142 km, of which approximately 60 km allow navigation. The San Carlos River flows from the union of the Jabillos and Peje rivers and flows into the San Juan River, near the community of Boca San Carlos. The tributaries of the San Carlos River are the Platanar, Peñas Blancas, Arenal, Tres Amigos, and the Hidalgo and Grande rivers, which are joined by the Aguas Zarcas and Sahíno rivers.

In the area it is common to fish for Machaca, Moga fish, Guapote, Guabina, Sabalo Real, among others.

In the Boca Tapada area, the traditional sport fishing tournament of Boca Tapada de Pital has been held consecutively for 20 years and its income is used to help the Pital Home for the Elderly.

Additionally, the river is an ideal place to observe crocodiles, turtles, bats, herons, kingfishers, vultures and many other species.