Gudbrandsdalen (Urban East Norwegian: [ˈɡʉ̂ː(d)brɑnsˌdɑːɳ]; English: Gudbrand Valley) is a valley and traditional district in the Norwegian county of Innlandet (formerly Oppland county). The valley is oriented in a north-westerly direction from Lillehammer and the lake of Mjøsa, extending 230 kilometers (140 mi) toward the Romsdalen valley. The river Gudbrandsdalslågen (Lågen) flows through the valley, starting from the lake Lesjaskogsvatnet and ending at the lake Mjøsa. The Otta river which flows through Otta valley is a major tributary to the main river Lågen. The valleys of the tributary rivers such as Otta and Gausa (Gausdal) are usually regarded as part of Gudbrandsdalen. The total area of the valley is calculated from the areas of the related municipalities. Gudbrandsdalen is the main valley in a web of smaller valleys. On the western (right hand) side there are long adjacent valleys: Ottadalen stretches 100 kilometers (62 mi) from Otta village, Gausdal some 50 kilometers (31 mi) from Lillehammer and Heidal some 40 kilometers (25 mi) from Sjoa. Gudbrandsdalen runs between the major mountain ranges of Norway including Jotunheimen and Dovrefjell–Rondane.
Together with the Glomma river and the Østerdalen valley, the river Lågen and the Gudbrandsdalen valley form Norway's largest drainage system covering major parts of Eastern Norway. Gudbrandsdalen is home to Dovre Line railway and the European route E6 highway. The valley is the main land transport corridor through Eastern Norway, from Oslo and central eastern lowlands to Trondheim and Møre og Romsdal.