Østerdalen (Urban East Norwegian: [ˈœ̂sːtəˌɖɑːɳ])[1] is a valley and traditional district in Innlandet county, in Eastern Norway.

The more mountainous northern half of Østerdalen includes the municipalities Rendalen, Alvdal, Folldal, Tynset, Tolga and Os.

The more forested southern half includes the municipalities Elverum (which is also the district's largest town), Stor-Elvdal, Engerdal, Trysil and Åmot.

Typical of Østerdalen is that large parts of the forest floor are covered with reindeer moss, a variety of lichen.

The name was referring to the valleys of the rivers Glomma, Rena, Trysilelva, and Österdalälven (the parishes Idre and Särna belonged to Norway until 1644).