A mindset training aims to strengthen the belief that abilities are malleable (growth mindset), which has proven to be beneficial for learning. Teachers can support the effects of such a training by establishing a classroom culture in line with the growth mindset idea. Yet, previous training programs have mostly been detached from regular lessons. In this study, a physics teacher implemented a mindset training that consisted of explicit training sessions and implicit training phases. In these implicit phases, the teacher enriched ordinary lessons with growth mindset feedback. We investigated the overall effect of this lesson-integrated training on students’ beliefs and motivation. Students from two seventh-grade courses participated in the quasi-experimental study ( N = 59). One course received the mindset training; the other course served as control. We measured growth mindsets about physics abilities, self-beliefs, and motivation before and after the training and six months later. The results indicate that there was a positive and stable training effect on growth mindsets, but no effect on self-beliefs. Regarding motivation, the training buffered the demotivation that occurred without training. We conclude that a mindset training is important when introducing a new and difficult school subject. Furthermore, we consider teachers’ involvement as a promising approach to optimize mindset interventions and to encourage a sustained change of instructional practices.