Lough Derg or Loch Derg (Irish: Loch Dearg)^[1] is a lake in County Donegal, Republic of Ireland. It is near the border with Northern Ireland and lies about 7 kilometres (4.3 mi) north of the border village of Pettigo. It is best known for St Patrick's Purgatory, a site of pilgrimage on Station Island in the lake.

The lake is about 890 hectares (2,200 acres) in size, but is quite shallow, making it dangerous during bad weather.^[2] It has stocks of pike, perch and brown trout for angling.

The traditional three-day pilgrimage follows a one-thousand-year-old pattern. It begins on any day between June the first and August the 13th and lasts three days during which participants may only have one Lough Derg meal each day (black tea/coffee, dry toast, oat cakes, water). On arrival on the island, participants remove footwear and socks before commencing vocal prayers, walking around the island. A 24-hour night vigil then takes place on the first night. Generally, pilgrims depart on the morning of the third day having slept on their second night. They complete their pilgrimage fast at midnight the day of departure.^[3] The pilgrimage is suitable for persons over 15 years. Pilgrims must be able to walk and kneel unaided and be physically able to undertake the fast.

Including Station Island, there are about 30 islands and islets in Lough Derg, including:^[4]