Penrhyn Castle is a country house in Llandygai, 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 built between about 1822 and 1837 to designs by Thomas Hopper on behalf of its owner, who expanded and transformed the building beyond recognition. The owner of the castle had many slaves, and was compensated for being deprived of them after the abolition of slavery with about the same sum as construction of the castle had cost.