Leonardus ("Leen") Eustachius Jansen (3 August 1930 – 27 January 2014) was a 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.

His last fight was on 2 October 1967 in his home town of Rotterdam, where he was defeated on points by Johnny Halafihi of Tonga.

[3] On 27 January 2014, Jansen died in the hospital of Bergen op Zoom at the age of 83.

This biographical article related to a Dutch boxer is a stub.