Penningby Castle is a fortified manor in Uppland. Construction probably began in the late 15th century; its original stone house may have been built for Nils Bosson Sture. The castle took its current scale under Lars Turesson, who owned it from 1543 to 1560 and added diagonal gun towers designed for flanking fire.

The castle stands on a rocky point beside a lake that formed part of an old water route from the Baltic Sea into inland Uppland. Its lower floors remain largely unchanged, while the main floors were remodeled in the 1730s and 1810s. In the late 17th and early 18th centuries, the upper floors were altered with a mansard roof, larger windows and a new room layout.

A fire in 1831 destroyed the roofs and upper sections. The building was simplified rather than restored to its 18th-century appearance. The main section has kept its post-fire gabled roof, while the towers regained domes during restoration work in 1951–53. Penningby has been a listed building since 1980. It was also used as a filming location for several productions, including Mai Zetterling’s films Nattlek and Amorosa.