Grand Morin, River system in Seine-et-Marne, France.

The Grand Morin is a 118 km river that rises near Lachy in the Marne department and flows west into the Marne at Condé-Sainte-Libiaire, east of Marne-la-Vallée. Its winding course crosses 43 km in the Marne and 77 km in Seine-et-Marne, splitting into two arms at several points such as Coulommiers and La Ferté-Gaucher to form small islands. The river drops from 185 m at its source to 44 m at its confluence, with a gentle gradient of 1 to 2 m per km.

During the medieval period, the river powered a series of mills that processed grain for the surrounding communities, making it central to the local economy. Villages grew along its banks over the following centuries, and the stone bridges built to cross it are among the oldest surviving structures in the area.

Several old mills still stand along the banks, visible reminders of how the river shaped life in the villages for generations. Fishing remains a common weekend activity, and locals can often be seen casting lines from the banks or from small bridges.

The river runs east to west for 118 km, from its source near Lachy to its confluence with the Marne at Condé-Sainte-Libiaire, east of Marne-la-Vallée. The N34 road follows much of its course, and between La Ferté-Gaucher and Coulommiers the D66 hugs the river's loops closely. The gradient is gentle, one to two metres per kilometre, so the only drops are mill weirs, passable in a light boat.

The Grand Morin is the largest river of the Brie region after the Marne, and its valley gave its name to one of the Brie's terroirs, "la Brie des Morins". Its east-west course has shaped settlement since Antiquity, guiding roads and the finger-like urban spread along the valley. Its northern twin, the Petit Morin, carries roughly half its average flow.