Sihlwald, Protected forest reserve in Zürich, Switzerland.

Sihlwald is a roughly 12 km² forest reserve on the eastern slopes of the Albis hills above the Sihl river, owned by the city of Zürich but lying in the municipalities of Horgen, Adliswil, Langnau am Albis and others. It is the largest unmanaged contiguous mixed deciduous forest in the densely settled Swiss Plateau. European beech makes up 39% of the trees, Norway spruce 25%, alongside silver fir, ash, yew, pine, sycamore and wych elm.

Zürich acquired the forest on the left bank of the Sihl over centuries by binding local farmers into paid service obligations, and in 1524 the dissolution of the Fraumünster convent brought the right-bank woodland to the city. The Sihlamt was set up in 1424 to oversee timber use and rafting, and felling ceased in the 1990s under forest director Andreas Speich. On 28 August 2009 the Sihlwald became Switzerland's first nationally recognised nature discovery park.

The forest reflects how people have shaped it and how it recovers on its own. Visitors can see the transition from managed woodland to wild forest happening in real time as they walk through different areas.

The Visitor Center opens during warmer months and offers a bookstore, exhibition space, a playground, and fireplaces with free firewood available. The hiking trails are well marked and suitable for different fitness levels.

Rangers regularly patrol the forest and explain to visitors how natural processes unfold without human disturbance. This living laboratory makes the difference between wild and managed woodland immediately understandable.