Dr. Jan Constant Janssen (18 September 1895 – 15 April 1970) was a Belgian physician and businessman.
He started his career as a family physician in Turnhout, but in addition he was interested in business too.
In 1938 he closed his practice as a family physician and concentrated his effort of the expansion of his pharmaceutical business.
Still a medical student, his son Paul Janssen helped with the development of Perdolan.
After World War II, the brandname of the products was changed into Eupharma, and later the company would evolve into Janssen Pharmaceutica.