Classroom environments need to provide young children with opportunities to practice self-regulation in order to develop social and emotional competence. The effects of a mindfulness-based program on self-regulation, prosocial behavior and hyperactivity were examined in a study of 127 children (ages 4–6) in 8 kindergarten classrooms that were randomly assigned to either a Mindfulness Group (n = 72) or to a Control Group (n = 55). The program consisted of 20-minute lessons, delivered 3 times a week, for 6 weeks. The Head-Toes-Knees-Shoulders (HTKS) was used as a direct performance-based measure of self-regulation. The teacher version of the Strengths and Difficulties Questionnaire (SDQ) was used to assess prosocial behavior and hyperactivity. Data were collected before (Time 1) and after (Time 2) the 6-week period of the study intervention. Results indicated that children in the Mindfulness Group showed greater improvement in self-regulation, F (1, 124) = 10.70, p = .001 (=.079), were more prosocial (z = −4.152, p < .001) and less hyperactive (z = −3.377, p = .001) compared to children in the Control Group at Time 2. This was especially true for children who had lower scores at Time 1 (HTKS; rho = −.551, p < .001; Prosocial Behavior scale; rho = −.69, p < .001 and the Hyperactivity scale; rho = −.39, p < .001). Results highlight the benefits of mindfulness-based programs in kindergarten classrooms and indicate that they are particularly effective for children with difficulties in these areas.