Background: Individuals in secure mental health units experience disproportionately high rates of overweight and obesity, significantly increasing their risk of metabolic syndrome and premature morbidity. Although the Make Every Contact Count (MECC) initiative encourages health promotion conversations, there is currently no structured guidance to support nurse-led weight discussions in secure settings.

Aims: This study aimed to co-design an intervention with service users and nursing staff to facilitate structured, person-centred conversations about weight in low- and medium-secure mental health inpatient units.

Methods: A three-stage, Person-Based Approach (PBA) was adopted. Stage 1 involved a systematic review and qualitative interviews with service users (n=20) and nursing staff (n=10) to identify barriers and enablers to weight management conversations. Stage 2 aligned with PBA and co-design principles, which led to significant stakeholders’ involvement in developing a tailored intervention: the Nurse Led Weight Talk Tool. Stage 3 employed the think-aloud method with 19 service user–nurse pairs to assess the tool’s acceptability and usability.

Results: Barriers to weight discussions included competing service demands, relational dynamics, institutional restrictions, and perceived role limitations among nurses. Enablers included the need for brief, structured tools to support self-management and facilitate safe, meaningful conversations. The tool was co-designed to be visually engaging, values-based, and easily integrated into routine care. Think-aloud feedback led to refinements in language, layouts, and usability features.

Conclusions: This study demonstrates the potential of structured, nurse-led interventions to support weight management in secure settings. Developed through co-design and informed by behavioural change frameworks and implementation science, the Weight Talk Tool addresses individual, interpersonal, and systemic barriers.

Implications: Brief, theory-informed, evidence-based, and person-based interventions can empower nurses to engage service users in sensitive discussions about weight, contributing to more equitable, health-promoting care within secure mental health inpatient settings.