Castlewellan is a village in County Down, Northern Ireland. It is beside Castlewellan Lake and Slievenaslat mountain, southwest of Downpatrick. It lies between the Mourne Mountains and Slieve Croob. It had a population of 2,392 people in the 2001 Census.
Castlewellan has a wide main street which runs through two main squares lined with chestnut trees. The town was designed by a French architect for the Annesley Family. The Annesley family did not always own the land as they bought it off The Maginess Family. (see Earl Annesley), then owners of what is now Castlewellan Christian Conference Centre and Forest Park, and is unique within Ireland due to its tree lined squares both in the old town (upper square) and new town (lower square) as well as its very wide main street. The old market house in the upper square was built in 1764 and now houses the public library.