Kvennsjøen is a lake in the municipality of Ullensvang in Vestland county, Norway. The lake lies on the Hardangervidda plateau, inside the Hardangervidda National Park. The alpine lake is located straight east of the town of Odda and about south of the village of Eidfjord. There are over 500 lakes in Ullensvang municipality and this one is the largest of all of them.