King John's Castle also known as Limerick Castle is a 13th-century castle located on King's Island in Limerick, Ireland, next to the River Shannon. Although the site dates back to 922 when the Vikings lived on the Island, the castle itself was built on the orders of King John of England in 1200. One of the best preserved Norman castles in Europe, the walls, towers and fortifications remain today and are visitor attractions. The remains of a Viking settlement were uncovered during archaeological excavations at the site in 1900.

King's Island is an area of central Limerick, Ireland. The area is often referred to colloquially as The Island and consists of two distinct areas, Englishtown and St Mary's Park. The historical city of Limerick known as Englishtown is located on the southern end and St Mary's Park; a local authority housing estate is located on the northern end.