A new model of the human psyche is proposed comprising four structural components and two forms of mindfulness. First, the 'Subselves' are theorised to be the source of one's constant 'mindchatter' and the cause of much of the mental disturbance encountered in psychotherapy especially in depression. The second structural component is the 'Managing-ego' which is responsible for everyday interaction with other people and with the outside world. The third component is the 'Inner-observer' which is posited as one's 'true self' able to passively watch one's mental processes but without any active intervention. The final component is the 'Unconscious' which is acknowledged but is not further explored in the present model. The tetradic model also integrates mindfulness into the dynamics of the psyche. Specifically, it postulates that 'active mindfulness' operates in conjunction with the Managing-ego whereas 'passive mindfulness' occurs only when the Inner-observer is neutrally engaged as a watcher 'above mind'. Also, the construct of 'anti-mindfulness' is introduced to explain how mindchatter can completely block one's mind from employing any effective form of mindfulness. Further, two therapeutic mechanisms are identified, namely, decentering and disidentification. Decentering entails the Managing-ego using active mindfulness to separate oneself from the content of one's own thoughts. By contrast, disidentification can result from the passive mindfulness of one's Inner-observer enhanced by stillness meditation practice. The counselling technique of Voice Dialogue is discussed as an effective tool for applying the model. Finally, three diverse case examples are presented illustrating how the tetradic model can be applied in psychotherapeutic practice.