San Carlos is the capital city of the municipality of San Carlos and of the Río San Juan Department of Nicaragua.
The city proper has a population of roughly 12,174, while the city and surrounding communities (Comarcas) contain 37,461 as of 2005. San Carlos is positioned on the confluence of Lake Nicaragua and the San Juan River. The Solentiname Islands in Lake Nicaragua are part of the Municipality of San Carlos.
The municipality is divided into the following comarcas:
| San Isidro | San Carlos | Santa Isabel | Lauren Galán | Cruz Verde |
| México | Mata de Caña | La Azucena | Palo de Arquito Arriba nº1 | Palo de Arquito Arriba n°2 |
| Los Chiles | La Trinidad | Esperanza nº1 | La Culebra | La Argentina |
| Caño Luis | Morrillo | Solentiname | Las Maravillas | Papaturro |
| Valle de Guadalupe | Nueva Armenia | La Trinidad | Las Minas | Isla Medio Queso |
| El Consuelo | La Unión | San José | La Venada | Río Frío |
| El Corozal | Guapinol | Santo Domingo | San José Morrillo | Ojo de Agua |
| Espavel | Costa Sur | Camibar | Jumusa | Papaturito |
| Pueblo Nuevo nº1 | Pueblo Nuevo nº2 | El Plomo | Flor de Mayo | El Cairo |
| Guacalito | La Argentina | San Vicente | Nueva Jerusalén | El Consuelo nº1 |
| El Consuelo nº2 | Loma Quemada | Laurel nº2 | Quinta Lidia | Empalme Cruz Verde |
| Danubio | Monte Piedad | El Papayo | El Pavón nº1 | El Pavón nº2 |
| Las Palmeras | Las Venturas | San Agustín | La Bodega | Poza Redonda nº1 |
| Poza Redonda nº2 | San Ramón | San Antonio | San Pablo | Nueva Armenia |
| El Fajardo | Melchorrita |
San Carlos is twinned with:
| Municipality | |
 |
|
 San Carlos
|
|
Coordinates:  11°08′N 84°47′W |
|
| Country |  Nicaragua |
|---|---|
| Department | Río San Juan Department |
| Area | |
| • Municipality | 564 sq mi (1,462 km2) |
| Population (2005) | |
| • Municipality | 37,461 |
| • Urban | 21,111 |
