Weissenau Castle, Medieval castle ruin in Unterseen, Switzerland.

Weissenau Castle is a castle ruin in Unterseen situated along the shore of Lake Thun with remnants from different periods. The standing tower, partially collapsed walls, and sections of the original curtain wall form the visible remains of this former fortress.

The fortress was first recorded in 1298 and functioned as a defensive structure protecting mountain routes. It gradually lost importance during the 16th century and eventually fell into ruins.

The name Weissenau comes from the castle's first written record of 1298, castrum Wissenowe. The market town of Widen grew across the channel and was linked to the castle by a bridge in 1362; when Interlaken Monastery moved the weekly markets and yearly fair to Aarmühle in 1365, Widen declined. The ruin has been a protected monument since 1981 and was conserved in 1988–89.

The ruins are accessible via walking paths through the protected park area along Lake Thun's shore and can be explored on foot. The site offers open access with good views of the lake and surrounding landscape.

The castle originally stood on an island at the mouth of the Aare River and was gradually connected to the mainland through natural sediment buildup over centuries. This geological transformation makes the location singular in its landscape history.