The Hers-Mort (the "Dead Hers", as opposed to the faster-flowing Hers-Vif, or "Live Hers") is a long river in southern France, a right-bank tributary of the Garonne. Its average flow rate is . The Hers-Mort rises in the Lauragais region, near the village Fonters-du-Razès, in the Aude department. It flows northwest through the following departments and towns:
- Aude: Payra-sur-l'Hers, Salles-sur-l'Hers
- Haute-Garonne: Villefranche-de-Lauragais, Baziège, Toulouse, Saint-Jory
It flows into the Garonne near Grenade-sur-Garonne. Its waters, augmented by the Girou which flows into its right bank, irrigate the market gardens around Toulouse. The Canal du Midi crosses the Hers-Mort near Villefranche-de-Lauragais via the Lers Aqueduct.