Schlei, Baltic Sea inlet in Schleswig-Holstein, Germany

Schlei is a Baltic Sea inlet in Schleswig-Holstein that stretches 32 kilometers from Kappeln to the city of Schleswig. The water narrows in places to a few hundred meters wide and forms numerous coves along its winding course.

Hedeby, at the head of the inlet, was a Viking trading settlement that was later abandoned in favour of Schleswig. A museum now stands on the Hedeby site and tells the story of the abandoned town. The inlet's inner basin, the Burgsee with Gottorf Castle, was cut off from the Schlei in 1582 by Duke Adolf I with the Gottorf Dam, now about 28 metres wide and nearly 100 metres long.

The name Schlei probably comes from the Danish word slæ, meaning reeds or water plants, and was once used only for the inner broads near Schleswig. The Norse word angr, "narrow", gave the Angeln peninsula its name. A few professional fishermen still work the water, and the Schleischnäpel, a whitefish related to trout, is a local speciality.

Boat rental stations operate in several towns along the shore, where docking points for private boats are also available. The best water access is found in the larger settlements on both shores.

The waterway separates the Angeln peninsula to the north from Schwansen to the south, creating a natural boundary. Extensive reed belts grow along the shores and provide habitat for waterfowl.