Burg Ottenstein, Medieval castle in Rastenfeld, Austria.

Burg Ottenstein is a granite castle built on a rocky spur above the Ottenstein reservoir in the Waldviertel region of Austria. The complex includes several towers, thick defensive walls, and a small chapel inside the fortified enclosure.

The castle was built in the 12th century to control the trade route along the Kamp River. Over time it shifted from a military stronghold into a residential seat for the local nobility.

The castle chapel, dedicated to St. Florian, was decorated with extensive stucco work by Lorando Aliprandi. In the so-called Päpstezimmer (Popes' Room) hang portraits of 241 popes. The castle's former art collection, assembled by the Lambergs, was donated in 1822 by Franz Adam von Lamberg to the Academy of Fine Arts in Vienna.

The castle is privately owned and not generally open to visitors; the former castle restaurant in the outer bailey closed in 2014 and now operates only for pre-booked groups and weddings. The grounds can be viewed from outside, and the castle stands on a hill above the Ottenstein reservoir.

The castle chapel, one of the oldest parts of the complex, holds frescoes from the second half of the 12th century that were only uncovered in 1975. In the so-called Päpstezimmer, portraits of 241 popes hang on the walls.