Recently, several top athletes, as well as coaches, have revealed that mindfulness can be regarded as a type of psychological training to optimize performance. Indeed, results of studies in clinical psychology, as well as neuroscience, suggest that mindfulness might be an effective strategy to promote concentration, emotion regulation and state of flow. However, the empirical evidence in the context of sport is rather sparse. The aim of this study was to examine the effectiveness of a mindfulness-based intervention in athletes. In a randomized controlled trial with 22 participants in the experimental and 24 participants in the control group, the effectiveness of the Berlin Mindfulness-based Training for Athletes (BATL) was tested and compared to a classical sport psychological intervention. The results of an analysis of covariance with repeated measures indicate that the intervention group significantly improved mindfulness as a trait compared to the control group. It can be concluded that BATL is an effective strategy to increase mindfulness in athletes. However, further studies are needed to evaluate the effectiveness of this intervention in improving performance and to be able to examine its effect mechanisms.