Nordmannslågen is a lake in the municipality of Eidfjord in Vestland county, Norway. At 11.09 km2, it is the largest lake in Hordaland county. The lake lies on the great Hardangervidda plateau in the central part of the Hardangervidda National Park. The lake lies about 35 km southeast of the village of Eidfjord and about 50 km northeast of the town of Odda. The lake is part of the headwaters of the river Numedalslågen.