Newcastle (Irish: An Caisleán Nua) is a village in County Wicklow, Ireland. It is situated on the R761 and is about 45 km (28 mi) south of Dublin city. Newcastle had a population of 951 in the village and hinterland in the census of 2011. The ancient barony of Newcastle, which surrounds the village, takes its name from the village and its Norman castle.

The village is half a mile from the castle and church where it was originally located. Newcastle takes its name from the castle, built by the Normans on an earlier Irish fortification in the territory of the O’Byrne’s. It was constructed between 1177 and 1184 by Hugh de Lacy, Lord of Meath. The castle, called Newcastle Mackynegan, became a major stronghold in the outer fortifications of the Pale. Even so, it was attacked and occupied at intervals by the Wicklow Irish chieftains the O’Tooles and the O’Byrnes. The current ruin on the site is not that of the castle which was destroyed in the 16th century but of a building which was erected on the site. The castle was the administrative centre of the area until the shiring of Wicklow in the 16th century. The county seat then moved to Wicklow town after the castle was again raided.