Aims: The aim of this study was to explore perspectives on motivations for treatment engagement from substance use disorder (SUD) clients in a long-term residential rehabilitation program. Design and Methods: A convenience sample of 30 clients who were enrolled in a year-long SUD treatment program at a residential rehabilitation facility took part in in-depth interviews. Interview transcripts were analyzed using the directed content analysis approach. Results: Participant accounts indicated that their treatment engagement was motivated by factors that aligned with the six primary constructs of the Health Belief Model: (i) perceived susceptibility (eg, believing that their substance use required intervention and that they were prone to relapse), (ii) perceived severity (eg, substance use negatively impacted their health and harmed their close relationships), (iii) perceived benefits (eg, opportunities for a better life, reconnecting with family members and close friends, & avoiding legal consequences), (iv) perceived barriers (eg, the length of the treatment program), (v) cues to actions (eg, decisive moments, elements of the treatment program, & faith and spirituality), and (vi) self-efficacy in remaining abstinent (eg, treatment program provided them with skills and experiences to maintain long-term sobriety). Discussion: Our analysis indicates that participants’ treatment engagement was linked to their beliefs regarding the severity of their substance use disorder, their treatment program’s ability to help them avoid future relapse, and their own capability to act upon the strategies and resources provided by the treatment program. A theoretical understanding of these aspects can contribute to the future planning of precision interventions.