Burg Rennenberg, Medieval castle ruins near Linz am Rhein, Germany.

Burg Rennenberg is a ruined hilltop castle on a 349.7 m basalt knoll about 3.5 km northeast of Linz am Rhein, between the Losbach and Rennenberger Bach valleys. Its round bergfried, built of basalt rubble, has a lower section from the 14th century and a narrower upper section added in the 19th century. South of the tower lie the remains of a 30 m by 15 m residential building with walls up to 1.2 m thick.

The castle was first documented in 1217, when Konrad von Rennenberg and his sons appear as witnesses in a charter of Cologne Archbishop Engelbert I. In the mid-13th century it belonged to Countess Mechthild of Sayn, who agreed in 1250 that it would pass to the Archbishopric of Cologne after her death; by 1321 it was a Cologne fief. It was described as decayed by 1585.

The fortress was home to the Rennenberg family and symbolized the power of Rhine nobility throughout the area. It represented the rule of a local noble family whose influence shaped the region for centuries.

The ruin stands on a 349.7 m basalt knoll about 3.5 km northeast of Linz am Rhein's old town and 2 km west of Notscheid. No restoration work has been carried out since 2007, so the walls and the ring wall with its narrow zwinger should be left as found. The keep survives to roughly 7 m; the terrain around the neck ditch on the north side is steep.

The upper, narrower section of the round basalt keep was added in the 19th century, when Prince Friedrich IV of Salm-Kyrburg built Schloss Rennenberg in the Rennenberger Bachtal about two kilometres to the south-west and had the partly collapsed keep heightened. The lower masonry dates from the 14th century; the tower now stands about 7 m tall.