Peel Castle is a castle in Peel, Isle of Man originally constructed by Vikings. The castle stands on St Patrick's Isle which is connected to the town by causeway. It is now owned by Manx National Heritage and is open to visitors.

The castle was built in the 11th century by the Vikings, under the rule of King Magnus Barelegs. While there were older stone Celtic monastic buildings on the island, the first Viking fortifications were built of wood. The prominent round tower was originally part of the Celtic monastery, but has had battlements added at a later date. In the early 14th century, the majority of the walls and towers were built primarily from local red sandstone, which is found abundantly in the area. After the rule of the Vikings, the castle continued to be used by the church due to the cathedral built there - the see of Sodor Diocese - but was eventually abandoned in the 18th century. The castle remained fortified and new defensive positions were added as late as 1860. The buildings within the castle are now mostly ruined, but the outer walls remain intact.

Excavations in 1982-87 revealed an extensive graveyard as well as the remains of Magnus Bareleg's original wooden fort. The most spectacular finds were the 10th century grave of "The Pagan Lady" which included a fine example of a Viking necklace and a cache of silver coins dating from about 1030.

The Castle's most famous "resident" is the so called Moddey Dhoo or Black Dog ghost.

Peel Castle is sometimes confused with Piel Castle, located on Piel Island, around 30 miles to the east in the Irish Sea.This particularly occurs in reference to the William Wordsworth poem describing Piel, spelling its name as 'Peele'. Further confusion is added by the fact that Wordsworth is documented as having visited Peel Castle, and wrote about the Isle of Man on a number of times.