Work-related stress is normal and at low levels, it can actually increase productivity. However, high levels of stress can lead to a decrease in performance at work and can also cause adverse effects to health. High stress has been associated with cardiovascular diseases, sleep deprivation and anxiety disorder. Because of this, it is important to be able to cope with stress everyday to reduce its negative cumulative effects. In this paper, we describe the design, implementation, and evaluation of a mobile Just-in-Time Adaptive Intervention (JITAI) system for the treatment of psychological stress. The system has three main components: a stress recognition module, a set of stress interventions, and an intervention recommender system. The recognition module uses a Microsoft Band 2 to detect heart rate and a total of 18 stress micro-interventions are managed by the intervention recommender system. Finally, the recommender system uses a modified version of Q-learning to adapt the treatments to the needs and context of each patient.