Schleswig-Flensburg is a Kreis (district) in the northeastern part of Schleswig-Holstein, Germany. It borders on Denmark in the north, and the Baltic Sea in the east. Area 2070 km², population 198,000 (2002). The district's capital is Schleswig.

The district was established in 1974 by merging the former districts of Flensburg-Land and Schleswig. The city of Flensburg is not a part of this district. Due to the neighbourhood of Denmark and the regional history there is a large percentage of Danish inhabitants.

The countryside is generally plain. The Schlei, a firth (Förde) of the Baltic Sea, is the southern border of this district. All the land north of the Schlei and south of Flensburg is called the peninsula of Angeln (Anglia). Anglia was the ancient home of the Germanic people known as the Angles, which migrated to Britain in the early Middle Ages.

Written history in the area began about 800 AD, when the Viking settlement of Haithabu was founded. Later the neighbouring city of Schleswig took the place of Haithabu and became a powerful town in the 11th century. It later lost its power to Lübeck.

Towns in Schleswig-Flensburg: Schleswig, Kappeln, Glücksburg, Arnis.