In Roman city planning, a Decumanus Maximus was an east-west-oriented road in a Roman city, military camp, or colonia. The main decumanus was the Decumanus Maximus that normally connects the Porta Pretoria (in a military camp, closest to the enemy) to the Porta Decumana (away from the enemy). In the middle the Decumanus Maximus crosses the perpendicular Cardo or Cardus Maximus, the primary north-south road that was the usual main street. The Forum is normally located at this intersection of the Decumanus Maximus and the Cardus Maximus.

The term 'decumanus' is derived from the east-west line the augurs would draw when making the auspices.