The Thuringian Forest (Thüringer Wald in German pronounced [ˈtyːʁɪŋɐ ˈvalt] ^ⓘ) is a mountain range in the southern parts of the German state of Thuringia, running northwest to southeast. Skirting from its southerly source in foothills to a gorge on its north-west side is the Werra valley. On the other side of the Forest is an upper outcrop of the North German Plain, the Thuringian Basin, which includes the city of Erfurt. The south and south-east continuation of the range is the highland often called the Thuringian-Vogtlandian Slate Mountains.

Among scattered foothills at its northern foot are the towns Eisenach, Gotha and Arnstadt. The towns of Ilmenau and Suhl sit in slight dips on the range itself to the north and south respectively.

The Thuringian Forest forms a continuous chain of ancient rounded mountains with steep slopes to both sides and poses ample difficulties in transit routing save through a few navigable passes. It is about 70 km (43 mi) long and 20 km (12 mi) wide. The highest elevation is Großer Beerberg at 982 m (3,222 ft) above sea level.

The Rennsteig (sometimes called Rennweg) is an ancient path following the main ridge and connecting the summits. It is now a famous hiking path and marks the traditional boundary between the hills-dominated terrain of central Germany and the more rugged terrain characteristic of southern Germany, and also the boundary between the cultural regions of central and north Thuringia and Franconia. Dialects and traditional customs and costumes are different on either side of the Rennsteig. The Rennsteig is the subject of the song Rennsteiglied (de), the unofficial hymn of Thuringia.

Motorway A 4 passes north of the Thuringian Forest, while A 71, intersecting the former south of Erfurt, crosses the range from the northeast to the southwest, passes under the ridge in the Rennsteig Tunnel near Oberhof, and is joined near Suhl by A 73. Two more long-distance roads, Bundesstraßen 19 and 84, pass over the western parts of the range, while Bundesstraße 88 skirts the northern foothills between Eisenach and Geraberg.

The Neudietendorf–Ritschenhausen railway crosses the Thuringian Forest in Brandleite Tunnel between Gehlberg and Oberhof, the Werra Railway between Eisenach and Eisfeld does so in a tunnel near Förtha. Both are in daily operation. A third line, the southern section of the Plaue–Themar railway, does not use a tunnel, but crosses the mountain ridge at Rennsteig switchback station. It has only been used by museum trains since 1998. The Nuremberg–Erfurt high-speed railway, due to be commissioned in December 2017, crosses the Thuringian Forest with the help of several tunnels and bridges.

Thüringerwaldbahn, a cross-country line of the Gotha tramway network, serves the northern foothills of the Thuringian Forest between Gotha and Bad Tabarz, including a branch to Waltershausen.

Geologically, the Thuringian Forest is defined by a belt of strongly uplifted and deformed metamorphic and igneous rock that divides the relatively flat sedimentary plains of the Thüringer Becken (to the northeast) from similar rock formations in the valley of the Werra (to the southwest). It consists of a large fault block in hercynian orientation, which consists from sandstones and conglomerates of Rotliegend age in its western parts (Eisenach trough), followed by granites and gneisses of the Ruhlaer Kristallin formation of early paleozoic origin which were uplifted in the Rotliegend era, and the conglomerates, sandstones, and abundant volcanic rocks (rhyolites and andesites) of the Oberhof trough. Ore deposits associated with the upthrust of the range have been of significant historical importance in the development of the region, for example, the metalworking tradition in Suhl and the mining history of Ilmenau.