Leonardus ("Leen") Eustachius Jansen (born August 3, 1930 in Rotterdam, Zuid-Holland) is a former boxer from the Netherlands, who competed for his native country at the 1952 Summer Olympics in Helsinki, Finland. There he was stopped in the quarterfinals of the Men's Middleweight (–75 kg) division by eventual winner Floyd Patterson of the United States.

After the Summer Olympics Jansen became a professional. He boxed a total number of 71 matches, from which he won sixty fights (27 ko's). His last fight was on October 2, 1967 in his home town of Rotterdam, where he was defeated on points by Johnny Halafihi of Tonga.
Source : Wikipedia