Judah ben Isaac Messer Leon (1166–1224) was a French tosafist born in Paris.
Born in 1166 in Paris, France, his father Rabbi Isaac Treves was a rabbi in Paris, son of Yom Tov of Falaise who was the son of Judah ben Nathan (Rivan), and thus a descendant of Rashi.
In a list of that year associated with Abraham occurs the name of Leo Blund, whom Jacobs identifies with Judah ben Isaac.
[2] Sir Leon must have left Paris in 1182, when all Jews were expelled from the French king's dominions; he did not return until 1198.
A long fragment of his tosafot to Abodah Zarah is still extant in a manuscript that formerly belonged to Samuel David Luzzatto and Solomon Joachim Halberstam 'R.E.J.