Objectives: This systematic review identifies and synthesizes quantitative evidence regarding whether psychosocial constructs, specifically groupness, social support, self-efficacy, enjoyment, and instructor behavior, are associated with behavioral adherence to adult group fitness classes. Methods: Electronic searches were conducted in PubMed, Scopus, and Web of Science. Eligible studies included quantitative analyses of adults in instructor-led group exercise. Risk of bias was appraised using ROBINS-I, and JBI critical appraisal tools, as appropriate to study design. Results: Twenty-one studies met inclusion criteria. Self-efficacy was the most consistently supported domain, with several behavior-specific efficacy measures showing small-to-moderate positive associations with attendance-based adherence, although null findings were also observed when measures were less temporally aligned or when outcomes shifted to post-program continuation. Social-relational and instructor-related constructs showed more context-dependent evidence: social attraction, social cohesion, reliable alliance, group identification, and selected leader behaviors were more consistently favorable than generic social support or broad cohesion measures. Enjoyment and affective response showed the weakest independent evidence, with positive associations mainly when affective constructs were measured close to the attendance decision. Adherence in instructor-led group exercise settings appears to be influenced by perceived capability, social-relational processes, instructor-shaped motivational climate, and affective valuation, although the available evidence is heterogeneous and predominantly observational. Future research should use multilevel longitudinal designs, instructor- and class-level clustering, repeated affective assessment, and causal pathway modeling to test how these mechanisms support repeated attendance and longer-term maintenance.