The Rennsteig is a ridge walk as well as an historical boundary path in the Thuringian Forest, Thuringian Highland and Franconian Forest in central Germany.

In the Middle Ages the Rennsteig marked the boundary of the Duchy of Franconia with the Landgraviate of Thuringia. Even today it forms a clear border between the Franconian part of South Thuringia from the otherwise overwhelmingly Thuringian-Upper Saxon state of Thuringia. The Rennsteig is the language border separating the East Franconian dialects of Hennebergisch, Itzgründisch and Upper Franconian from the Thuringian dialects of Central Thuringian, Ilm Thuringian and Southeastern Thuringian, which is spoken in the mountains, in northern parts and east of the Thuringian Forest. The Rennsteig is also the watershed between the river systems of the Werra/Weser, Saale/Elbe and Main/Rhine. The catchment areas of all three river systems meet at the Dreistromstein (Three Rivers Stone) near Siegmundsburg.

Today the Rennsteig is designated by the German Mountain and Hiking Club (Verband Deutscher Gebirgs- und Wandervereine) as an outstanding high-grade hiking trail. It was discovered by August Trinius for the hiking fraternity and became famous well beyond the borders of Thuringia and Franconia mainly through the publications of the Rennsteig Club founded in 1896. The Rennsteig Cycle Way was opened on 19 June 2000. Most of it is provided with a water-bound surface, some sections also run along quiet country lanes. Here and there it departs from the course of the historic Rennsteig, so that steep inclines may be avoided. It is therefore about 30 km longer than the hiking trail.