Alte Sorge-Schleife, Nature reserve in Schleswig-Holstein, Germany

Alte Sorge-Schleife is a 758-hectare nature reserve in the municipalities of Meggerdorf, Bergenhusen and Erfde, northwest of Rendsburg. It covers a stretch of the Alte Sorge, an old meander arm cut off from the river, with pronounced bends, wet grassland and narrow reed belts, plus the Colsrakmoor bog west of Meggerdorf. Water levels are held back to flood wide grassland areas shallowly.

The Alte Sorge-Schleife was first protected by ordinance on 19 September 1991, when the reserve covered about 660 hectares. A new designation on 25 September 2009 replaced it with a reserve of roughly 758 hectares, registered as number 144 in Schleswig-Holstein's reserve list. In June 2012 a management plan was drawn up for the reserve as part of the Natura 2000 sites "Moore der Eider-Treene-Sorge-Niederung" and "Eider-Treene-Sorge-Niederung".

The reserve forms part of a regional conservation program for the wet lowlands between Eider, Treene and Sorge rivers, serving as an educational resource for understanding local wetland ecosystems.

The reserve is best explored using marked trails and observation points, with the breeding season from March to August offering the most active wildlife viewing. Wet conditions mean sturdy footwear and weather-appropriate clothing are essential year-round.

The Colsrakmoor, a degraded raised bog within the reserve, is left entirely to natural development. Wanderwege cross parts of the moor and adjacent grassland, and the Eider-Treene-Sorge-Radweg runs along the former Rendsburg–Husum railway embankment that cuts through the area in the south.