The southern section of Route 125 runs parallel to Autoroute 25 in Laval, Mascouche, and Terrebonne.

At Saint-Esprit, the Autoroute ends, and 125 continues northwards until the entrance to the Mont Tremblant Provincial Park north of Saint-Donat.

It then crosses the Île Jésus ( Laval ) and the Mille Îles River on the Sophie-Masson Bridge to arrive in Terrebonne.

Once past the city of Mascouche, it looks again like a narrow secondary road and does not even have a double yellow line.

At the junction of Route 339, it takes it to form a small concurrency with the A-25 at its northern end.