The Petit Morin is an 86 km-long (53 mi) river in France, a left tributary of the Marne. Its source is in the commune Val-des-Marais. Its course crosses the departments of Marne, Aisne and Seine-et-Marne. It flows westwards through the towns of Montmirail, Villeneuve-sur-Bellot, Saint-Cyr-sur-Morin and Jouarre, finally flowing into the Marne in La Ferté-sous-Jouarre.