France's second-longest river (after the Loire), the Seine then flows 776 kilometres (482 mi) before it passes between the coastal communes of Le Havre and Honfleur, on the Normandy coast, into the English Channel.

Source-Seine borders the communes of Frôlois to the north-west, Chanceaux to the north, Poncey-sur-l'Ignon to the north-east, Bligny-le-Sec to the south-east, Salmaise to the south and Boux-sous-Salmaise to the south-west.

In the late 4th century AD, Roman Emperor Theodosius I ordered the closure of pagan temples at the Seine's source and gave their property to Christian institutions.

In accordance with this edict, in the 5th century the abbey of Sainte-Marie-de-Cestra, the closest religious institution to the Seine's source, received a donation from the Roman government.

After Saint-Germain-Source-Seine and Blessey merged, following a local consultation, the prefectoral decree and the Journal Officiel de la République Française referred to the commune as Source Seine [5] (without a hyphen), which was against the rules regarding the naming of political and administrative entities.