Castletown is a town in the Isle of Man, geographically within the historical parish of Malew but administered separately. Lying at the south of the island, it was the Manx capital until 1869. The centre of town is dominated by Castle Rushen, a well-preserved medieval castle, originally built for a Viking king.

Castletown is the former capital of the Isle of Man and site of the Tynwald, and can trace its roots back to 1090. The town has narrow streets and small fishing cottages. Castle Rushen (at the centre of the town) was originally built in 1265 for a Norse king, then fortified and added to by successive rulers between the 13th and 16th centuries. The castle has been used as a fortress, a residence for the Kings and Lords of Mann, the site of a mint and even a prison (past prisoners include a bishop and two newspaper editors). The town and castle were the site of a number of sieges and battles, especially during the years when control of the island passed between the Norse, Scots and English. Robert the Bruce laid siege to and captured the castle three times.

The history of the town and island is illustrated in four Manx National Heritage sites in the centre of Castletown: Castle Rushen, the Nautical Museum (in the secret passage-filled home of inventor, politician, banker and probable smuggler George Quayle), the Old Grammar School (originally a church from 1200 AD) and the Old House of Keys.

Fishing boats still go out to fish from the harbour. Commercial traffic to the port ended in the 1970s, although there has been an ongoing expansion of financial and industrial businesses in the area. The first telephones on the Isle of Man appeared in Castletown in 1901.