Hoan Kiem Lake (Vietnamese: Hồ Hoàn Kiếm, meaning "Lake of the Returned Sword" or "Lake of the Restored Sword", also known as Hồ Gươm - Sword Lake) is a lake in the historical center of Hanoi, the capital city of Vietnam. The lake is one of the major scenic spots in the city and serves as a focal point for its public life.

According to the legend, emperor Lê Lợi handed a magic sword called Heaven's Will which brought him victory in his revolt against the Chinese Ming Dynasty back to the Golden Turtle God (Kim Qui) in the lake and hence gave it its present name (the lake was formerly known as Luc Thuy meaning "Green Water"). The Turtle Tower (Thap Rùa) standing on a small island near the centre of lake is linked to the legend.

Large soft-shell turtles, either of the species Rafetus swinhoei or a separate species named Rafetus leloii in honor of the emperor, have been sighted in the lake. The species is critically endangered and the number of individuals in the lake is unclear.

Near the northern shore of the donkey lies Jade Island on which the Temple of the Jade Mountain (Ngoc Son Temple) stands. The temple was erected in the 18th century. It honors the 13-century military leader Tran Hung Dao who distinguished himself in the fight against the Yuan Dynasty, Van Xuong, a scholar, and Nguyen Van Sieu, a Confucian master and famous writer in charge of repairs made to the temple in 1864. Jade Island is connected to the shore by the wooden red-painted The Huc Bridge (The Huc, meaning Morning Sunlight Bridge).