The Southern Interior Mountains ecoprovince is in the southeast corner of the province, and is home to large mountain ranges, thick forests and rich wetlands. The region is the territory of several First Nations including the Ktunaxa, Kinbasket, Kalispell, Stoney, Secwepemc, Saulteaux, Cree, Sinixt, and Dakelhne.

Several large national parks cover parts of the Columbia Mountains and the Rockies, and these areas get huge snowfalls in the winter. In general, the winters are long and the summers are short.

Both coniferous and deciduous forests cover the mountain slopes and the valleys. Rivers and wetlands are found throughout the valleys in the area, and many of the ducks and other migratory birds in B.C. can be found in these waters.

The Flathead River Valley is one of the most biodiverse areas in the ecoprovince and in North America. It is rich in plant species and is an important habitat to animals like the grizzly bear.

The major cities are Cranbrook, Nelson, Revelstoke and Trail.