San Marcos is bordered on the northeast by the department of Huehuetenango, on the east by the department of Quetzaltenango, to the south by the department of Retalhuleu and by the Pacific Ocean, and on the west by the Mexican state of Chiapas.

The department has a total territory of 2,397 square kilometres (925 sq mi).

The northern portion of the department is mountainous, being crossed by the Sierra Madre de Chiapas mountain range.
The two highest volcanoes in Central America being located within its borders,  Tajumulco (4,220 metres (13,850 ft) high) and Tacaná (4,092 metres (13,425 ft) high).
The Tajumulco volcano is the highest peak in Central America.
Other notable peaks include San Antonio Ichiguán at 4,022 metres (13,196 ft) above mean sea level.
The southern portion of the department drops away towards the Pacific coast.

The Tacaná volcano, which occupies the border with Mexico and has slopes in both countries, has been historically active, with activity recorded in 1855, 1878, from 1900 through to 1903, from 1949 to 1950 and from 1986 to 1987.

Principal rivers in the department include the Suchiate River, which marks the border between San Marcos and Chiapas, Mexico, and flows into the Pacific Ocean.
The Cabuz River has its source on the slopes of the Tajumulco volcano.
Other rivers include the El Pajapa, El Rodeo, Ixlamá, Ixtal, Meléndrez, Nahuatán, Tilapa, and Ocosito Rivers.

The climate varies widely within the department, due to the dramatic differences in altitude, with the north possessing a cold Guatemalan Highlands climate and the south having a temperate or hot climate, depending on the specific altitude.
San Marcos is bordered on the northeast by the department of Huehuetenango, on the east by the department of Quetzaltenango, to the south by the department of Retalhuleu and by the Pacific Ocean, and on the west by the Mexican state of Chiapas.
The department has a total territory of 2,397 square kilometres (925 sq mi).
The northern portion of the department is mountainous, being crossed by the Sierra Madre de Chiapas mountain range. The two highest volcanoes in Central America being located within its borders,  Tajumulco (4,220 metres (13,850 ft) high) and Tacaná (4,092 metres (13,425 ft) high). The Tajumulco volcano is the highest peak in Central America. Other notable peaks include San Antonio Ichiguán at 4,022 metres (13,196 ft) above mean sea level. The southern portion of the department drops away towards the Pacific coast.
The Tacaná volcano, which occupies the border with Mexico and has slopes in both countries, has been historically active, with activity recorded in 1855, 1878, from 1900 through to 1903, from 1949 to 1950 and from 1986 to 1987.
Principal rivers in the department include the Suchiate River, which marks the border between San Marcos and Chiapas, Mexico, and flows into the Pacific Ocean. The Cabuz River has its source on the slopes of the Tajumulco volcano. Other rivers include the El Pajapa, El Rodeo, Ixlamá, Ixtal, Meléndrez, Nahuatán, Tilapa, and Ocosito Rivers.
The climate varies widely within the department, due to the dramatic differences in altitude, with the north possessing a cold Guatemalan Highlands climate and the south having a temperate or hot climate, depending on the specific altitude.
San Marcos Department