Grebiner See, Schluensee und Schmarkau, Special Area of Conservation in Schleswig-Holstein, Germany.

The FFH-Gebiet Grebiner See, Schluensee und Schmarkau is a 241-hectare Natura 2000 site in the Schleswig-Holstein hills, split into three separate parts. The northern part is the Grebiner See with its shore north of Grebin; the largest is the Schluensee between Kossau and Grebin, just east of the B 430; the southern part is the lower Schmarkau from the Schmarksee to its mouth in the Behler See. Two thirds of the area is inland water, with beech woods on the Sophienwarder peninsula and in the Hohen Köhlen forest.

The area was initially proposed as significant for nature in 2004 and gained official protection status as a European conservation site in 2007. This recognition led to its full designation in 2010 as a Natura 2000 site to ensure long-term preservation of its valuable habitats.

The site lies entirely within the Naturpark Holsteinische Schweiz. The Schluensee section also falls inside the Landschaftsschutzgebiet Trammer See, Schluensee, Wald- und Knicklandschaft zwischen Schöhsee und Behler See, while the Schmarkau section partly overlaps the Landschaftsschutzgebiet Holsteinische Schweiz. As of November 2022, no institution had been commissioned to manage the area under § 20 LNatSchG.

Visitors can walk on marked trails around the lakes and along the river, but must respect protected zones where wildlife needs to remain undisturbed. The area is best explored on foot or by bicycle, and it is wise to dress for the weather since wetland conditions vary by season.

The Schluensee's nutrient-poor, calcareous waters are recorded as habitat type 3140, characterised by stoneworts (Armleuchteralgen). This is one of five habitat types listed in the site's 2019 standard data form, alongside beech forests on the Sophienwarder peninsula and in the Hohen Köhlen woods.