Jan Dekert or Jan Dekiert (1738 – 4 October 1790) was a Polish merchant of German descent and political activist. Starting in the 1760s, he rose to become one of the most prominent merchants in the Polish capital of Warsaw. He was an activist arguing for more rights for the burghers in the Polish–Lithuanian Commonwealth while opposing Jewish merchants. As the representative of Warsaw, he was elected a deputy to the Sejms of 1784 and 1786, as well as to the Great Sejm (1788–1792). He was the mayor of Warsaw (1789–1790), during which period he organized the Black Procession on 2 December 1789 (a march of burghers who delivered a petition to the king). This was a major step towards the passing of the Free Royal Cities Act enfranchising burghers, as one of the reforms of the Great Sejm and part of the Constitution of the 3rd May, 1791.