Fontus Lake is the oval-shaped 150 m long in south-southwest to north-northeast direction and 100 m wide lake in the middle part of South Beaches on Byers Peninsula, Livingston Island in the South Shetland Islands, Antarctica. It has a surface area of 1.9 ha and is separated from sea by a 53 to 64 m wide strip of land. The area was visited by early 19th century sealers.