Idstein is a town of about 25,000 inhabitants in the Rheingau-Taunus-Kreis in the Regierungsbezirk of Darmstadt in Hesse, Germany. Because of its well preserved historical Altstadt (Old Town) it is part of the Deutsche Fachwerkstraße (German Timber-Frame Road), connecting towns with fine fachwerk buildings and houses. In 2002, the town hosted the 42nd Hessentag state festival.

Idstein lies in the Taunus mountain range, about 16 kilometres (10 miles) north of Wiesbaden. The town's landmark is the Hexenturm (Witches' Tower), a 12th-century bergfried and part of Idstein Castle.

The Old Town is found between the two brooks running through town, the Wolfsbach in the east and the Wörsbach in the west, on a high ridge reaching up to 400 m (1,300 ft) above sea level. This comes to an end in the Old Town's north end with the castle and palace crags, behind which the two brooks run together. On the Wolfsbach, remnants of the like-named, now forsaken village can still be made out. The estate agent Gassenbach in the town's south goes back to an old settlement called Gassenbach; for the last few years, it has belonged to the Domäne Mechtildshausen, an organic farming operation.

West of town, beyond the Wörsbach valley, lies another high ridge with peaks ranging from the Hohe Kanzel (592 m or 1,942 ft) to the Roßberg (426 m or 1,398 ft) and the Rügert (402 m or 1,319 ft) to the Rosenkippel (379 m or 1,243 ft); to the south, the Galgenberg (348 m or 1,142 ft) forms another high area over to the Dasbach Heath. Just under the western heights run the Autobahn A 3 and the Cologne-Frankfurt high-speed rail line (in the Idsteintunnel along the slope).

On the other side of the Rügert are the constituent communities of Oberauroff and Niederauroff in the valley of the Auroffer Bach.

North of Idstein, the Wörsbach valley reaches into the Goldener Grund, fertile cropland that stretches all the way to the Lahn valley.