More than a quarter of all Germans are affected by a mental illness. They are usually treated through psychotherapy, a bilateral or group process between client and therapist. However, partners, relatives and close friends of psychotherapy clients are largely excluded. They often report difficulties in understanding what the person is going through and how they can best help. However, the link between social support and mental health is well established. CoTherapy will be the first digital support solution focused on fostering psychotherapy clients' support system, strengthening relationships and facilitating social support as a key factor in therapy outcomes.