Johannes Janssonius (1588 – 1664; born Jan Janszoon, in English also Jan Jansson) was a Dutch cartographer and publisher who lived and worked in Amsterdam in the 17th century.
Janssonius was born in Arnhem, the son of Jan Janszoon the Elder, a publisher and bookseller. In 1612 he married Elisabeth de Hondt, the daughter of Jodocus Hondius. He produced his first maps in 1616 of France and Italy.
In the 1630s he formed a partnership with his brother in law Henricus Hondius, and together they published atlases as Mercator/Hondius/Janssonius. Under the leadership of Janssonius the Hondius Atlas was steadily enlarged.
After Janssonius’s death, the publishing company was continued by his son-in law, Johannes van Waesbergen. The London bookseller Moses Pitt attempted publication of the Atlas Major in English, but ran out of resources after the fourth volume in 1683.