Tarn is one of the original 83 departments created during the French Revolution on 4 March 1790, through application of the Law of 22 December 1789.

Tarn's three principal ranges lying to the south-east are: the Mountains of Lacaune, the Sidobre and the Montagne Noire, belonging to the Cévennes.

The highest point of the range and of the department is the Puech Montgrand, 1,267 m (4,157 ft) high; several other summits are not much short of this.

The Montagne Noire, on the southwestern border of the department, derives its name from the forests on its northern slope.

The limestone and sandstone foot-hills are clothed with vines and fruit trees, and are broken by deep alluvial valleys of particular fertility.

With the exception of a small portion of the Montagne Noire, which drains into the river Aude, the whole department belongs to the basin of the Garonne.