Hoan Kiem 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 was boating on the lake when a Golden Turtle God (Kim Qui) surfaced and asked for his magic sword, Heaven's Will. Lợi concluded that Kim Qui had come to reclaim the sword that its master, a local God, the Dragon King (Vietnamese: Long Vương) had given Lợi some time earlier, during his revolt against the Chinese Ming Dynasty. Lợi renamed the lake to commemorate this event, it 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 lake 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 13th-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).