Jean Sylvain Bailly (15 September 1736 – 12 November 1793) was a French scientist and political leader during the French Revolution and the mayor of Paris from 1789 to 1791.

Considered too conservative, Bailly would be succeeded as mayor of Paris by Jérôme Pétion de Villeneuve and was eventually sentenced to death after refusing to testify against Marie Antoinette. After being taken to the Champ de Mars, he was tortured by a frenzied mob for two hours in a ditch before being guillotined. It was said that Bailly's execution was "intended as an expiatory ceremony".