This article describes a secondary data analysis collected from inmates who participated in an independent randomized controlled trial, testing the efficacy of the Growing Pro-Social (GPS) Program. The current study aimed to test the program’s ability to increase, on one hand, cognitive reappraisal (adaptive emotion regulation strategy) and, on the other hand, decrease expressive suppression (maladaptive emotion regulation strategy) over time. It was also assessed if the GPS was capable of reducing disciplinary infractions committed by inmates over time. Participants were randomized to the GPS treatment (n = 121) or the control group (n = 133). The Emotion Regulation Questionnaire was completed at baseline, at mid-treatment, at post-treatment and at 12-months’ follow-up. Disciplinary infractions were collected from prison records during the 12 months before the beginning of the program, during the GPS’s 12-month length and during the 12 months after treatment completion. Treatment effects were analyzed with latent growth curve models. Concerning cognitive reappraisal, while treatment participants showed a significant increase, controls presented a decrease over time. For expressive suppression, the treatment group presented a significant decrease, and the control group showed no change over time. Treatment participants also presented a significant decrease in the number of disciplinary infractions and in the number of days in punishment, while controls showed no change or an increase over time. This study showed the GPS’s ability to promote emotion and behavior regulation, which contributes not only to inmate’s interpersonal adjustment, but also to a more efficient management of the prison system.