Penningby Castle, Medieval castle in Länna, Sweden.

Penningby Castle consists of a nearly square main building with two diagonally placed round corner towers, the southwest one containing a large well. The two lower floors are built of grey stone with barrel-vaulted rooms, while the two upper floors, probably rebuilt in the 17th century, are of sandstone. It stands by Penningbyån and Väsbysjön, waterways that were part of the route between the Baltic and Uppsala.

The fortress was founded in the 1440s by noblewoman Birgitta Tordsdotter Bonde and shaped the region's early settlement history. Major renovations in the 1550s added an eastern entrance, transforming its layout and defensive capabilities.

The round salon in the sea tower displays decorative details and architectural features from the medieval and Renaissance periods. These spaces show how residents lived and worked across different eras of the castle's occupation.

The castle stands by the Penningbyån stream and Väsbysjön lake in Länna parish, Norrtälje Municipality, about 70 km north of Stockholm. It is privately owned and no longer inhabited; the buildings are shown to visitors in summer. The medieval core sits on a strategic spot by waterways that once linked the Baltic coast with the Uppsala area.

Olof Persson Stille, one of the first Swedish settlers of New Sweden (later Pennsylvania), worked at Penningby before leaving for America. The castle also served as a film location: Mai Zetterling shot Nattlek there in 1965 and Amorosa in 1985, and the TV series Huset Silfvercronas gåta was filmed there in 1973.