Primary objective: To explore the experiences of program mentors, participants, and employees involved in a peer-based physical activity (PA) program for adults with moderate-to-severe TBI, being piloted by a community fitness center, to develop the program as a measurable intervention.

Research design: We adopted an exploratory case study approach through an interpretivist paradigm, which focused on discovering realities about the peer-based PA program across the study participants' views, backgrounds, and experiences.

Methods and procedures: Semi-structured focus groups and individual interviews were conducted with nine adult program participants (3 peer mentors, 6 participants), and three program employees. Inductive content analysis was used to develop themes about their perceived experiences.

Main outcomes and results: 44 open-codes were grouped into 10 subthemes and three final themes: 1) program impacts identified the importance of the program in daily life and resulting psychological, physical, and social outcomes; 2) program characteristics highlighted program leaders, accessibility, and social inclusion; 3) program sustainability included program adherence, benefits for the center, and the program's future.

Conclusions: Perceptions of program experiences and outcomes identified how peer-based PA for adults with moderate-to-severe TBI can lead to meaningful activities, functioning better, and buy-in from all parties. Implications for research and practice related to supporting health-related behaviors after TBI through group-based, autonomy-supporting approaches are discussed.