Opaliński castle in Sieraków, Medieval castle museum in Sieraków, Poland.

Opaliński castle in Sieraków is a former residence with stone walls and architectural elements from multiple building periods. The southern wing operates as a museum displaying collections related to the region's development and heritage.

A fortified stronghold existed here from 1397. In the 17th century the site passed to the Opaliński family, who rebuilt it as a residence; the stone walls and mixed architectural elements visible today date from these successive building campaigns. The structure later lost its residential role and survives in part, with the southern wing now housing the museum.

The castle displays objects from different periods that tell the story of the region and its people. These items offer insight into daily life and the development of the area over centuries.

The castle remains stand in Sieraków, about 75 km northwest of Poznań, near the Sieraków Landscape Park. The museum entrance is on Stadnina street; guided tours of the collections and the surviving wings can be arranged. The site is on the edge of the town centre, within walking distance of the market square.

The tin and copper sarcophagi of the Opaliński family were found in 1991 during excavation of the former castle crypt. The metal coffins, dated to the 17th century, are displayed on the ground floor of the southern wing, now the museum.