John Duncan Gregory CB CMG (26 May 1878 – 29 January 1951) was a British diplomat. Gregory was the son of . He was educated at Eton College and joined the Diplomatic Service in 1902. He was promoted Second Secretary in 1902. From 1907 to 1909 he worked at the British Embassy in Vienna. He later became Chargé d'Affaires in Bucharest, was posted to the Holy See in 1915, and was promoted Counsellor in 1919. In 1920 he was posted to the Foreign Office in London as Senior Clerk and Assistant Secretary, and in 1925 he became Assistant Under-Secretary of State for Foreign Affairs.