Bucu (also known as Buku) is a hill island situated in Lübeck, Germany. It is geographically defined by its location surrounded by the Trave and Wakenitz rivers.
The site holds significant historical importance as the location of a medieval Slavic castle, which currently exists as ruins. In 1143, Count Adolf II of Holstein founded the city of Lübeck on this island. Today, the Burgkloster, or fortified monastery of Lübeck, is situated atop the remains of the Bucu ruins. The name Bucu is also used to refer to a hill.