The Osterburg is a castle from the High Middle Ages in the village of Groothusen in the municipality of Krummhörn, in the district of Aurich in Lower Saxony, Germany.

The village of Groothusen was originally built on a long warft in the early Middle Ages and was an important trading centre and meeting place in the Frisian Emsigerland, as well as the provost office for 12 parishes.

Around 1550 the castle was converted into a three-winged manor house, when two lower wings were built in front of the hall.

Today the Osterburg houses a significant collection of cultural and art historical artefacts such as furniture, weapons, a library, items of Old East Frisian culture and a remarkable art collection of ancestral portraits from five centuries.

The castle is now owned by the descendants of the chieftain family of Beninga and may be visited in guided tours arranged by telephone.