Riemeisterfenn, Nature reserve in Steglitz-Zehlendorf, Germany.

Riemeisterfenn is a 7.2-hectare nature reserve in Berlin's Steglitz-Zehlendorf district, on the edge of the Grunewald. It protects a marshy lowland around the largely silted-up Riemeistersee, with alder carr and nutrient-poor, acidic moor water. A canal on its north side links the Krumme Lanke with the Langes Luch.

Riemeisterfenn was placed under protection by regulation on 4 May 1987, covering 7.2 hectares of mesotrophic moor pond and alder carr. The Riemeistersee had dried out in 1911 after Berlin's groundwater extraction lowered the water table, and from 1913 Havel water was pumped into the Grunewald lake chain. A breach in a dam in the 1960s had reconnected the fen to the lakes; this link was closed again in 1998.

The name comes from the Low German word "Fenn" for a mire and from a "Riedmeister", an overseer appointed by Lehnin Abbey for the lakes and marshes; the word became Riemeister. The nearby Riemeisterstrasse and Riemeister primary school also carry the name. The Onkel-Tom-Strasse bordering the fen recalls the rustic excursion inn Onkel Toms Hütte, licensed in 1884.

The reserve is accessible by multiple hiking trails and connects to nearby lakes and other protected areas. Visitors should be prepared for wet ground and wear sturdy footwear.

The reserve preserves one of Berlin's last moorlands with rare plant species such as marsh orchids. This vegetation is a remnant from earlier times and survives only in a few city locations today.