Newcastle, New Brunswick was a former town on the Miramichi River in east central New Brunswick, Canada. Since its amalgamation with the downriver town of Chatham, New Brunswick and several smaller areas it has formed part of the city of Miramichi.

It was shire town of Northumberland County, and was the location of the court house and government offices. It was also the head of navigation on the river and had wharves for the export of lumber and other forest products. Its most prosperous days were before World War One. At its peak it had a population of about 6,500.

The British statesman and newspaper owner, Max Aitkin, Lord Beaverbrook, grew up here.