Competing against another individual for resources carries a cost that may or may not outweigh the benefit. The anterior cingulate cortex (ACC) has been implicated in cost–benefit decision-making, but it had not been evaluated in a competitive scenario. In this study, a confined rat and a freely moving rat had to compete for food. Recordings in the freely moving rat demonstrated that ACC neurons encode cost–benefit decisions that involve effort, indicating that the ACC registers competitive effort as a decision cost.