Burg Grub, Medieval castle ruin in Irnfritz-Messern, Austria.

Burg Grub is a castle ruin on a hilltop in Irnfritz-Messern from the Middle Ages. It consists of tower remains, palace walls, and a reconstructed chapel sitting at about 467 meters elevation.

Burg Grub is a hilltop castle from the 11th century near the village of Grub an der Großen Taffa, about 2 km southwest of Messern in the Waldviertel. In 1558 it passed to Veit Albrecht von Puchheim. Because it was no longer remodelled after 1500, it still shows its late medieval character, with the bergfried, remains of the palas and the rebuilt castle chapel.

Because the castle was not remodelled after 1500, it still shows its late-medieval character. The name Grub comes from the village of Grub an der Großen Taffa, about 2 km southwest of Messern. The site is a listed heritage monument (Denkmalgeschütztes Objekt).

The ruin is located at Grub 13 and can be viewed from the outside, with parking available for easy access. It is wise to wear sturdy shoes and explore the grounds with care.

In 1970 Franz Josef Hampapa (1914–2006) and his wife Maria Magdalena bought the ruin for 45,000 schillings to save it from demolition. With volunteers they secured many original building parts, and restoration was partly supervised by the Federal Monuments Office after advice from members of the Vienna University of Technology.