It flows in Drummond Regional County Municipality (MRC), north-west in the administrative region of Centre-du-Québec, on the South Shore of the Saint Lawrence River, in Quebec, Canada.

This river crosses the municipalities of: l'Avenir, Durham-Sud, Lefebvre, Wickham, Saint-Germain-de-Grantham and Drummondville.

The main neighboring hydrographic slopes of the Saint-Germain river are: The Saint-Germain river draws its water from the heads of various streams flowing in agricultural zones and crossing some forest islets, in the municipality of L'Avenir.

The head of the river is located 1.2 kilometres (0.75 mi) southwest of highway 55.

The toponym "Rivière Saint-Germain" was officially registered on December 5, 1968, at the Commission de toponymie du Québec.