Penrhyn Castle is a country house in Bangor, Gwynedd, North Wales, in the form of a Norman castle. It was originally a medieval fortified manor house, founded by Ednyfed Fychan. In 1438, Ioan ap Gruffudd was granted a licence to crenellate and he founded the stone castle and added a tower house. Samuel Wyatt reconstructed the property in the 1780s. The present building was created between 1820 and 1845 to designs by Thomas Hopper, who expanded and transformed the building beyond recognition. However a spiral staircase from the original property can still be seen, and a vaulted basement and other masonry were incorporated into the new structure. Hopper's clients were the Pennant family, who had made their fortune from Jamaican sugar and local slate quarries.

Penrhyn is one of the most admired of the numerous mock castles built in the United Kingdom in the 19th century; Christopher Hussey called it, "the outstanding instance of Normal revival." The castle is a picturesque composition that stretches over 600 feet from a tall donjon containing family rooms, through the main block built around the earlier house, to the service wing and the stables. It is in a very solid, sombre style which allows it to possess something of the medieval fortress air despite the ground-level drawing room windows. Hopper designed all the principal interiors in a rich but restrained Norman style, with much fine plasterwork and wood and stone carving. The castle also has some specially designed Norman style furniture, including a one ton slate state bed used by Queen Victoria when she visited in 1859.

In 1951 the castle and 40,000 acres (160 km²) of land were accepted by the Treasury in lieu of death duties. It now belongs to the National Trust and is open to the public. Penrhyn's attractions include a formal walled garden, extensive informal gardens, a dolls museum, an industrial railway museum, a model railway museum and an adventure playground. It has views over the Snowdonia mountains.