Johann Peter Theodor Janssen (12 December 1844, Düsseldorf – 19 February 1908, Düsseldorf) was a German historical painter.
Janssen was born in Düsseldorf, son of the engraver Tamme Weyert Theodor Janssen [de] (1817–1894), by whom he was first instructed before studying at the Kunstakademie Düsseldorf under Karl Ferdinand Sohn and Eduard Bendemann.
He is principally known through a series of decorative works whose monumental style and sound naturalism won him a reputation as one of the foremost historical painters of his time.
[citation needed] He became a professor at the Kunstakademie Düsseldorf in 1877 and its director in 1895, and was elected a member of the Prussian Academy of Arts in 1885.
He was the brother of sculptor Karl Janssen, whose works include the Kaiser Wilhelm Monument [de] in Düsseldorf, and "Steinklopferin" ("Stonebreaking Woman"), which is owned by the Alte Nationalgalerie in Berlin.