| Monte Tarn | |
| Other Name: | Cerro Tarn |
| Elevation M: | 825 |
| Location: | 70 km south of Punta Arenas, Chile |
| Range: | Andes |
| Coordinates: | -53.7853°N -71.0236°W |
| First Ascent: | February 1827 by John Tarn |
| Map: | Chile |
Mount Tarn is a small mountain located on the southernmost part of the Strait of Magellan, in Brunswick Peninsula, about 70 km south of Punta Arenas, Chile. It is in the southern extreme of continental Chile very close to Cape Froward, surrounded by historic places such as Fort Bulnes and Puerto del Hambre (Port Famine).
From the summit it is possible to see the Strait of Magellan, Dawson and Tierra del Fuego islands, and many other smaller ones; the Darwin Mountain Range, Mount Sarmiento, and most of the Brunswick Peninsula.
According to historian Mateo Martinic Beros in his book Cartografía Magallánica 1523-1945, the mount was named after the British surgeon, John Tarn, who first ascended the mountain in February 1827 while traveling with Robert FitzRoy on HMS Adventure and later ascended it while traveling with Phillip Parker King in HMS Beagle), during their surveying voyage from 1826 to 1830.
Tarn participated in a hydrographic survey conducted in the area, through the collection and classification of flora and fauna species.
On 6 February 1834 a group from the second Beagle survey expedition, including Charles Darwin, ascended Mount Tarn by forcing their way up through dense woodland to the bare ridge which took them to the summit. He recounted the story in his Journal and Remarks. In his ascent the young naturalist found the first ammonites ever known in South America.