Sierhagen, Heritage manor house in Altenkrempe, Germany.

Sierhagen is a manor house in Altenkrempe, Schleswig-Holstein, listed as a heritage monument. Its rectangular courtyard, about 100 by 50 metres, is enclosed by water-filled moats and four buildings, with gatehouses from 1738 and 1857. The three-storey neoclassical main house, rebuilt in 1825 by architect Alexis de Chateauneuf, stands slightly apart in the southeast. The estate is private and not open to the public.

Sierhagen was first mentioned in the 12th century as the Slavic noble castle Syra. Around 1316 it belonged to the knights Marquard and Eccard von Westensee. From the Middle Ages to the mid-17th century it was mostly held by the Buchwaldt family, followed by the counts Dernath in 1666 and the Brockdorff family in 1730. Wulf Heinrich von Thienen acquired it in 1765; through inheritance it passed to the Plessen family, who have owned it since 1809.

The estate has belonged to the Scheel-Plessen family since 1809; Carl von Scheel-Plessen (1811–1892), a German-Danish then Prussian politician, and his grandson Victor von Plessen (1900–1980), explorer and ornithologist, were born here. The family still runs the agricultural estate, and the private grounds occasionally host public events. The 1978 television adaptation of Christine Brückner's novel Jauche und Levkojen was filmed here.

The estate is private and not open to visitors; it only opens occasionally for public events. The four buildings stand around a courtyard enclosed by water-filled moats, reached through gatehouses built in 1738 and 1857. The Eisberg, a Neolithic burial mound, lies outside the estate grounds.

The estate's outer grounds hold a Neolithic burial mound called the Eisberg, separate from the walled courtyard. In 1978 the manor served as the filming location for the adaptation of Christine Brückner's novel "Jauche und Levkojen". The farm, about 1,650 hectares, is run by Carl-Christian von Scheel-Plessen.