Objectives <p>The COVID-19 pandemic necessitated the transition of Mindfulness-Based Stress Reduction (MBSR) from in-person to online delivery. However, the impact of this transition on participants’ characteristics, adherence, and outcomes in community-based MBSR programs has not been adequately examined. The present study retrospectively compared the effects on psychological well-being outcome measures of in-person delivery and online delivery without and with added individual interviews.</p> Method <p>A total of 506 Japanese participants completed validated questionnaires before and after participating in an 8-week community-based MBSR program delivered either in person (IP; <i>n</i> = 156) or online. Online participants attended the program either without brief individual interviews (OL-NoInt; <i>n</i> = 147) or in later cohorts with such interviews added to the protocol (OL-Int; <i>n</i> = 203). The questionnaires assessed three outcomes related to psychological well-being (PWB)—mood states, health-related quality of life, and perceived stress—and two presumed mediators of MBSR’s effects on PWB: mindfulness and self-compassion.</p> Results <p>Significant pre–post improvements were observed across all outcome measures in all three groups. Compared with IP, OL-NoInt demonstrated 32.3–62.0% smaller effect sizes in PWB outcomes despite comparable improvements in mediator variables. In contrast, OL-Int showed 27.0–184.2% larger effect sizes than OL-NoInt in both the mediators and the PWB outcomes, and was shown to be non-inferior to IP across all measures.</p> Conclusions <p>Online MBSR may be as effective as in-person delivery when the program is augmented with brief individual interviews. Findings underscore the crucial role of instructor factors in facilitating effective MBSR online delivery.</p> Preregistration <p>This study is not preregistered.</p>