Johannes Janssen (10 April 1829 – 24 December 1891) was a 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.
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