Mindfulness-based interventions for the prevention and treatment of depression are predicated on the idea that interoceptive awareness represents a crucial foundation for the cultivation of adaptive ways of responding to negative thoughts and mood states such as the ability to decenter. The current study used a multi-dimensional self-report assessment of interoceptive awareness, including regulatory and belief-related aspects of the construct, in order to characterize deficits in interoceptive awareness in depression, investigate whether brief mindfulness training could reduce these deficits, and to test whether the training unfolds its beneficial effects through the above-described pathway. Currently depressed patients (n = 67) were compared to healthy controls (n = 25) and then randomly allocated to receive either a brief training in mindfulness (per-protocol sample of n = 32) or an active control training (per-protocol sample of n = 28). Patients showed significant deficits across a range of regulatory and belief-related aspects of interoceptive awareness, mindfulness training significantly increased regulatory and belief-related aspects of interoceptive awareness, and reductions in depressive symptoms were mediated through a serial pathway in which training-related increases in aspects of interoceptive awareness were positively associated with the ability to decenter, which in turn was associated with reduced symptoms of depression. These results support the role of interoceptive awareness in facilitating adaptive responses to negative mood.