South Asian Americans experience multifaceted sociocultural and acculturative stressors that influence mental well-being, yet few studies have applied contemporary behavioral theories to understand relaxation behaviors in this population. This cross-sectional study examined predictors of initiating and sustaining relaxation behaviors using the Multi-Theory Model (MTM) of health behavior change. A web-based survey of 271 South Asian adults incorporated the Perceived Stress Scale (PSS-10), MTM constructs, and sociodemographic characteristics. Reliability was high across MTM subscales (Cronbach’s α = 0.81–0.93). Structural equation modeling demonstrated acceptable fit (CFI > 0.90, TLI > 0.90, RMSEA < 0.08, SRMR < 0.08). Hierarchical regressions revealed that among participants practicing relaxation (n = 202), behavioral confidence significantly predicted initiation (β = 0.481, p < 0.001), followed by participatory dialogue (β = 0.194, p < 0.05) and changes in the physical environment (β = 0.242, p < 0.01). Emotional transformation strongly predicted sustenance (β = 0.395, p < 0.001), along with practice for change (β = 0.307, p < 0.05) and changes in the social environment (β = 0.210, p < 0.05). MTM constructs explained 69.8% of initiation variance and 70.4% of sustenance variance. Among non-practitioners, participatory dialogue predicted initiation (β ≈ 0.18–0.34, p < 0.05), and emotional transformation predicted sustenance (β = 0.570, p < 0.001). These findings underscore MTM’s strong predictive utility and support culturally tailored interventions enhancing confidence, emotional regulation, and social/environmental supports to promote relaxation behaviors in South Asian communities in the United States.