Objectives: Developing the skills to positively manage social transgressions is of particular salience to those in the teaching profession. The Mindfulness-Based Emotional Balance (MBEB) program is a professional development program for K-12 teachers to build mindfulness and related prosocial skills such as empathy, compassion, and forgiveness. The present study assessed the acceptability of the MBEB program and tested whether MBEB was associated with changes in teachers' forgiveness skills. Methods: Participants included 171 teachers from 3 school districts who were randomized to the MBEB program or waitlist control group. Program attendance was recorded and MBEB teachers reported on the helpfulness of the forgiveness component. Five indices of forgiveness were assessed at pre-program, post-program, and in the fall of the subsequent school year (follow-up). A series of OLS regression models tested whether program condition was associated with the forgiveness outcomes at post-program and follow-up. Results: MBEB teachers attended 90% of sessions, and 70% of MBEB teachers reported the forgiveness component to be helpful or very helpful. In comparison to control teachers, MBEB teachers reported improvements in their efficacy to forgive colleagues and students, tendency to forgive, and situation-specific forgiveness at post-program. Changes in MBEB teachers' efficacy to forgive colleagues, tendency to forgive, and situation-specific forgiveness were sustained into the fall of the subsequent school year. Conclusions: Significant and sustained improvements in MBEB teachers' forgiveness skills indicate that teachers, who are actively engaged in a high-stress profession, can benefit from a mindfulness-based program aimed at cultivating prosocial qualities such as forgiveness. [This paper was published in "Mindfulness" v11 p1978-1992 2020.]