Úlfljótsvatn (Úlfljótur’s Lake or Ugly Wolf Lake) is a lake in southern Iceland, to the south of lake Þingvallavatn, 74km east of Reykjavík. Úlfljótsvatn is named after Úlfljótur, an important man who was involved in the Althingi (Icelandic Parliament) in 930. In the vicinity, the Icelanders have many summer cottages. At the deepest point, Úlfljótsvatn is about 20 meters deep.
The lake has a water area of 2.45 km² and is located directly south of the larger lake Þingvallavatn on the Sog river, which continues to Álftavatn. The lake attracts anglers in the summer season. Trout and arctic char can be found in the waters of the lake.