Johannes "Hans" J. G. Jansen (17 November 1942 - 5 May 2015) was a retired scholar of contemporary Islam and politician in the Netherlands.

Jansen was a graduate of the University of Amsterdam, his doctorate in Arabic is from the University of Leiden. He worked in Egypt, where he was the Director of the Dutch Research Institute in Cairo. From 1983 he was an associate professor of Arabic and Islamic studies at Leiden University. Up to 2008, he was Houtsma Professor of Modern Islamic Thought at the University of Utrecht.

He was a regular newspaper columnist and frequent media commentator. He was a principal witness in the trial of Geert Wilders, during which a judge approached him during a dinner party and allegedly tried to convince him that the trial was justified. As a result, the judges were substituted and a retrial was ordered. The judge who was alleged to have attempted to influence him subsequently retired from the bench.

He was an expert witness on the Koran and Shariah in the trial in Birmingham Magistrates Court (Crown vs Timothy Martin Burton) on 8 April 2014, which came to be known as the Birmingham Taqiyya Trial. (Mr Burton was subsequently found not guilty of Racially Aggravated Harassment.)

Until his death he was a Member of the European Parliament for the political party Party for Freedom.