Pfeffingen Castle, Castle ruin in Pfeffingen, Switzerland.

Pfeffingen Castle (Ruine Pfeffingen) is one of the largest castle ruins in the canton of Basel-Landschaft, standing on the ridge of the Blauen above the municipality of Pfeffingen. At its centre are the remains of the massive residential tower, surrounded by the 13th-century shield wall and ring wall, with traces of several towers, gateways and other buildings. The ruins are visible from the Birseck and from the Angensteiner Klus.

Pfeffingen Castle was first mentioned in 1135 with Notker von Pfeffingen and passed to the Counts of Thierstein around 1200. In the mid-13th century the castle was rebuilt with a curtain wall and a large residential tower. Basel besieged and burned it in 1376, and it was damaged by the 1356 Basel earthquake. After the Thierstein line died out in 1519, the castle went to the Bishop of Basel in 1522. It was abandoned around 1750 and auctioned for demolition in 1761.

The castle gave its name to the counts of Thierstein-Pfeffingen, whose line died out with Oswald II in 1513 and Wilhelm in 1519. A local legend tells of the miraculous rescue of Walram von Thierstein-Pfeffingen in the 1356 Basel earthquake. After the family's end, a hermit lived for a time in the abandoned buildings.

The ruin is reached by marked hiking trails and there is no admission fee. The path climbs uphill and the ground among the ruins is uneven, so sturdy footwear is recommended.

The 1356 Basel earthquake is one of the strongest ever recorded in central Europe, and this castle was among the structures it damaged. The marks left by that event can still be read in the surviving sections of wall.