<p>Diabetes and depression are both chronic conditions. They have a bi-directional relationship, with common drivers, and each worsens outcomes for the other. However, they tend to be treated in silos and the integration of mental and physical healthcare is rarely a priority. Behavioural activation is a simple therapy that has demonstrated portability and effectiveness for addressing depression in different contexts. The “Developing and evaluating an adapted behavioural activation intervention for people with depression and diabetes in South Asia” programme aims to address the gap in mental health treatment in physical health services in Bangladesh and Pakistan, through adapting and testing a behavioural activation intervention in diabetes services in both countries. As part of the programme a feasibility trial was conducted across four sites in Bangladesh and Pakistan. This paper describes the mixed-methods process evaluation of the feasibility trial examining context, implementation and mechanisms of action of the intervention. The results of the process evaluation are promising, with participants who received the intervention reporting behavioural activation to be helpful in both alleviating mood and management of diabetes. Fidelity to the intervention was high, though there were some differences across the four sites in terms of supervision and understanding of the intervention components. Participants reported an increase in knowledge (about depression and diabetes), motivation to engage in everyday activities and a positive relationship with the intervention facilitators. The results of the process evaluation have informed changes to the preparation and adaptations of the intervention for a full randomised controlled trial.</p>