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.

The valley is divided into three parts:

The municipalities within the valley fall under the Gudbrandsdal District Court. Until 2016, the valley was also its own police district. The Gudbrandsdalen district covers about 60% of the former Oppland county.

The invention of the modern, firm, fatty variant of Brunost cheese is commonly attributed to the milkmaid Anne Hov from the rural valley.

The main character in Henrik Ibsen's play Peer Gynt was inspired by a real or legendary person living in the valley in the 18th or 17th century. Ibsen travelled through the valley in 1862 and collected local stories, legends and poems. Ibsen also made drawings from his trip, including "Elstad in Gudbrandsdalen".

The name Gudbrandsdalen means 'the valley/dale of Gudbrand'. Gudbrand (Old Norse Guðbrandr) is an old male name compounded of guð, 'god' and brandr, 'sword'. The name may be derived from a chief (hersir) called Gudbrand. According Icelandic historian and poet Snorri Sturluson, the district was also referred to as i Dalom ("in the valleys") because of the many valleys. Dale-Gudbrand settled in Hundorp in the present-day Sør-Fron Municipality. At the time of Halfdan the Black there was a "chief Gudbrand north in Gubrandsdalen" (Gudbrand herse nord i Dalom). Later Eric Bloodaxe had an opponent called Dale-Gudbrand. According to the sage Olaf II of Norway met one Dale-Gudbrand, supposedly the last Dale-Gudbrand, in 1021. Historians believe there was a regional centre at Hundorp during the Viking era and that the name Gudbrand was used for many generations by the ruling family. Burial mounds (tumulus) at Hundorp suggest that powerful men are buried there.

The Gudbrandsdalen valley includes the most arid area in Norway. In Skjåk Municipality the average annual precipitation is only 278 millimetres (10.9 in). The valley sits in the rain shadow of the mountains west (including Jotunheimen), north, and east of the valley. The valley is less incised than the valleys of western Norway. Farming is mostly confined to the relatively narrow areas along the rivers. Gudbrandsdalen and adjacent valleys are surrounded by wide uplands and mountain plateaus traditional used as seter or summer farms.