Isteiner Klotz, Nature reserve in Efringen-Kirchen, Germany.

Isteiner Klotz is a 25.5-hectare nature reserve on the limestone outcrop of the same name, spanning the Efringen-Kirchen districts of Huttingen, Istein and Kleinkems. It runs from the Klotzenfelsen southward past the Grünberg to the Buchgraben and is cut in two by the Mannheim–Basel railway line. The rock faces are Malm limestone, and naturally treeless rock habitat now covers only about ten percent of the area.

The area held military importance during World War II because of its position close to the French border. Remains from that period, including bunkers, are still visible in the landscape today.

The name Istein comes from the former fishing village at the foot of the rock; the reserve's name refers to the Klotzenfelsen, also called Klotzenspitze. The dry grassland on the south-facing Malm limestone was first described in 1931 by botanist Braun-Blanquet, and it differs from similar grassland on the Kaiserstuhl. Visitors can look for feather grass, which has its only current occurrence in Germany here.

The area is accessible via several marked trails that begin from parking areas at the base. Information plaques along the routes explain the geology and the variety of plants and animals found here.

The dry grassland on the south-facing Malm limestone was first described in 1931 by the botanist Braun-Blanquet, who found it clearly different from the dry grasslands of the Kaiserstuhl. Among the plants here is feather grass, whose only occurrence in Germany is recorded at the Isteiner Klotz.