Hartwig Cassel (November 2, 1850 in Konitz, West Prussia (now Chojnice, Poland) – 1929) was a chess journalist, editor and promoter in Great Britain and the United States of America.
He lived in Scotland, where he was a member of the Glasgow Chess Club, and later moved to Bradford, Yorkshire, where he began his journalistic career as the chess editor of the Observer-Budget.
Cassel left England in 1889 and went to Havana for an English and New York newspaper syndicate to report the Mikhail Tchigorin-Gunsberg match.
He was instrumental in establishing the Staats-Zeitung and Rice trophies, and arranged, among other important contests, the first cable match between the Manhattan Chess Club and the British Chess Club in 1895, the forerunner of the Anglo-American series.
In 1904 he and Hermann Helms published the first issue of the American Chess Bulletin.