Sir Stephen Theodore Janssen, 4th Baronet (died 7 April 1777) was an English Member of Parliament and Lord Mayor of London. He was the 4th son of Sir Theodore Janssen, 1st Baronet and the younger brother of Sir Abraham Janssen, 2nd Baronet. He succeeded his other elder brother Sir Henry Janssen, 3rd Baronet to the baronetcy in 1766. He was a London stationer and master of the Stationer's Company from 1749 to 1751. He was elected an Alderman of the city of London in 1749 (until 1765), Sheriff of the City of London for 1749–50 and Lord Mayor of London for 1754–55.