Boo Castle, Heritage castle in Hallsberg Municipality, Sweden

Boo Castle is a stone building in Bo socken, Hallsberg Municipality, near Lake Avern, about 28 km southeast of Hallsberg. Its Gothic Revival front, built 1874–1882 to drawings by Johan Fredrik Åbom, has a stepped gable and a raised entrance portico with a balcony on columns. A round crenellated corner tower with a flagpole stands at the left, a taller octagonal tower with a pointed roof and finial at the right; tall arched windows sit above rectangular ones, behind a low stone wall.

The estate passed in 1692 from the siblings Anna and Gustaf Cruus to Anna Flemming (1683–1737), who married Baron Hugo Johan Hamilton in 1723; he built the Boo fideikommiss, a trust still in force and held by the Hamilton af Hageby family. Karl Didrik Hamilton rebuilt the Karoliner manor as a corps de logi in the 1780s. That house was demolished in 1878, and the present castle by Johan Fredrik Åbom went up in 1874–1882.

The interior displays artistic paintings and decorations from the late 19th century created by Carl Grabow. These artworks remain a defining feature of the rooms today.

The castle remains a private residence of the Hamilton af Hageby family, which limits public access to the building. Visitors should check in advance whether special tours or events might be available.

The Boo estate is one of Sweden's few remaining fideikommiss, a legal trust that keeps the property undivided and within the Hamilton af Hageby family since 1723. In the 1920s it covered about 16,500 hectares, and it still includes almost the entire parish of Boo.