Johannes Janssen (10 April 1829 – 24 December 1891) was a Catholic priest and German historian born in Xanten. After graduating from the Rektoratsschule in Xanten (today's Stiftsgymnasium), he was educated at the universities of Münster, Leuven, Bonn and Berlin, afterwards becoming a teacher of history in Frankfurt-am-Main.
He was ordained priest in 1860, became a member of the Prussian House of Deputies in 1875, and in 1880 was made domestic prelate to the pope and apostolic pronotary. He died at Frankfurt. Janssen was a stout champion of the Ultramontane party in the Roman Catholic Church.