Sir John Campbell Allen (October 1, 1817 – September 27, 1898) was from 1865–1896 a justice of the colonial and then provincial Supreme Court of New Brunswick, serving as Chief Justice of New Brunswick from 1875 to 1896.
He was born in Kingsclear Parish, New Brunswick, a grandson of Isaac Allen, a New Brunswick Supreme Court judge. He studied law in the office of John Simcoe Saunders.
He was a member of the New Brunswick House of Assembly from 1856 to 1865, Solicitor General in 1856 to 1857, Speaker of the House from 1863 to 1865, and Attorney General in 1865.
In 1873, he gave the majority decision of the New Brunswick Supreme Court in Dow v. Black, a significant constitutional law case dealing with the federal-provincial division of powers. He held that a provincial statute dealing with municipal taxation was unconstitutional. However, his decision was overturned on appeal by the Judicial Committee of the Privy Council, at that time the court of last resort for the British Empire.
John Campbell Allen is buried in the Forest Hill Cemetery in Fredericton.