The L Street Brownies are a polar bear club based in South Boston, Massachusetts. Organized in 1902, it is one of the oldest such clubs in the United States. Although the Brownies swim year round, they are best known for their annual New Year's Day plunge in Dorchester Bay.

A polar bear plunge is an event held during the winter where participants enter a body of water despite the low temperature. In the United States, polar bear plunges are usually held to raise money for a charitable organization. In Canada, polar bear swims are usually held on New Year's Day to celebrate the new year.