Established evidence reports that there is a bidirectional relationship between obesity and mental health, with obesity as both a cause and consequence of poor mental health. Treatments for obesity and poor mental health should account for this relationship, but often this is not the case. Behavioural weight management interventions are the most common form of obesity treatment across the United Kingdom. Whilst there is good evidence that these interventions can benefit physical health, the evidence for the effect on mental health is limited and unclear. By understanding how these interventions may impact mental health, potential adverse effects can be identified, and future intervention development informed to provide more effective support. The overall aim of this thesis was to investigate the role of participant mental health in adult behavioural weight management interventions. The specific objectives were (i) to determine the short- and long-term impact of behavioural weight management interventions on the mental health of adults with overweight and obesity; (ii) to identify how participant mental health is associated with attendance and engagement with a behavioural weight management intervention and trial; (iii) to identify participant characteristics associated with changes in mental health in adults with obesity during and after a behavioural weight management intervention; and (iv) to explore participant experiences of a behavioural weight management intervention to better understand how well the intervention supports participant mental health, and how the intervention could be adapted to provide more effective support. In Chapter Two, I conducted a systematic review and meta-analysis to assess the impact of behavioural weight management interventions on mental health in adults with overweight and obesity, compared to minimal intervention, ‘standard care’, or inactive control groups. I found evidence to suggest that, on average, interventions may benefit some aspects of mental health at intervention-end and 12 months from baseline. Notably, I found no evidence to suggest that interventions negatively impacted mental health (relative to comparator groups). In Chapter Three, I present the study details and participant characteristics from the Weight loss Referrals for Adults in Primary care (WRAP) trial, which acted as a data source for Chapters Four, Five, and Six. In Chapter Four, I conducted quantitative analyses to investigate whether participant mental health at baseline was associated with attendance and engagement with a behavioural weight management intervention and completion of follow-up assessments in the associated randomised controlled trial. I found evidence to suggest that intervention participants are less likely to attend intervention sessions, engage with intervention resources, and attend study follow-up visits if they report higher levels of depression or anxiety or lower scores for quality of life or satisfaction with life at baseline. In Chapter Five, I conducted quantitative analyses to assess the long-term impact of behavioural weight management interventions on symptoms of depression and anxiety. I found no evidence of a difference between intervention and control groups for changes in depression and anxiety symptoms at 5 years from baseline. On average, participants across all randomised groups did not experience meaningful changes (defined by minimal important difference) in depression and anxiety from baseline to 5 years. In Chapter Six, I conducted quantitative analyses to identify participant characteristics associated with changes in mental health during and after a behavioural weight management intervention. I found that those reporting higher baseline anxiety were likely to experience decreases in anxiety symptoms and increases in depression symptoms up to 5 years from baseline, whereas those reporting higher baseline depression were likely to experience decreases in depression symptoms and increases in anxiety symptoms up to 5 years from baseline. In Chapter Seven, I explored participants’ mental health experiences during a remote behavioural weight management intervention to support adults with overweight or obesity during the COVID-19 pandemic (SWiM-C). I conducted semi-structured interviews with twenty participants and used reflexive thematic analysis to identify patterns of meaning across the dataset relevant to mental health. Findings suggest that participants experienced multiple factors related to and external to the intervention that negatively impacted their mental health, yet aspects of the SWiM-C intervention appeared to support some participants to adaptively manage the decline in their mental health. As a whole, this thesis offers three main contributions to the field. Firstly, thesis findings suggest that, on average, behavioural weight management interventions appear to have net positive or neutral impacts on mental health. However, a proportion of participants do experience a decline in their mental health, and these appear to be participants beginning the intervention with poorer mental health. Next, my findings suggest that there may be a bidirectional relationship between intervention attendance/engagement and participant mental health. Specifically, findings suggest that i) poorer mental health at baseline is associated with lower rates of intervention attendance and engagement, and that ii) managing competing demands can lead to reduced intervention engagement which subsequentially worsens mental health. Finally, thesis findings suggest that results from trials of behavioural weight management interventions may be biased to those who are most mentally healthy at the beginning of the trial. The findings of this thesis contribute new insight into the role of mental health in adult behavioural weight management interventions. Thesis findings highlight a need for greater research into the role of mental health in behavioural weight management interventions; for example, greater research on i) the impact of interventions on mental health, ii) the mechanisms by which this impact may occur, iii) and approaches to better support participant mental health during weight management efforts would be of value.