Background: Meaning in life is positively associated with health, well-being, and longevity, which may be partially explained by engagement in healthier behaviors, including physical activity (PA). However, promoting awareness of meaning is a behavior change strategy that has not been tested in previous PA interventions.<br /> Objective: This study aims to develop, refine, and pilot-test the Meaningful Activity Program (MAP; MAP to Health), a web-based mobile health PA intervention, theoretically grounded in meaning and self-determination theory, for insufficient lyactive middle-aged adults. Methods: Following an iterative user-testing and refinement phase, we used a single-arm double baseline proof-of-conceptpilot trial design. Participants included 35 insufficiently active adults in midlife (aged 40-64 years) interested in increasing theirPA. After a 4-week baseline period, participants engaged in MAP to Health for 8 weeks. MAP to Health used a web-basedassessment and just-in-time SMS text messaging to individualize the intervention; promote meaning salience; support the basicpsychological needs of autonomy, competence, and relatedness; and increase PA. Participants completed measures of thehypothesized mechanisms of behavior change, including meaning salience, needs satisfaction, and autonomous motivation atpretest (-4 weeks), baseline (0 weeks), midpoint (4 weeks), and posttest (8 weeks) time points, and wore accelerometers for thestudy duration. At the end of the intervention, participants completed a qualitative interview. Mixed models compared changesin behavioral mechanisms during the intervention to changes before the intervention. Framework matrix analyses were used toanalyze qualitative data. Results: Participants were aged 50.8 (SD 8.2) years on average; predominantly female (27/35, 77%); and 20% (7/35) Asian,9% (3/35) Black or African American, 66% (23/35) White, and 6% (2/35) other race. Most (32/35, 91%) used MAP to Healthfor >= 5 of 8 weeks. Participants rated the intervention as easy to use (mean 4.3, SD 0.8 [out of 5.0]) and useful (mean 4.3, SD 0.6).None of the hypothesized mechanisms changed significantly during the preintervention phase (Cohen dvalues <0.15). However,autonomy (P<.001; Cohen d=0.76), competence (P<.001; Cohen d=0.65), relatedness (P=.004; Cohen d=0.46), autonomousmotivation (P<.001; Cohen d=0.37), and meaning salience (P<.001; Cohen d=0.40) increased significantly during the intervention.Comparison of slopes before the intervention versus during the intervention revealed that increases during the intervention weresignificantly greater for autonomy (P=.002), competence (P<.001), and meaning salience (P=.001); however, slopes were notsignificantly different for relatedness (P=.10) and autonomous motivation (P=.17). Qualitative themes offered suggestions forimprovement. Conclusions: MAP to Health was acceptable to participants, feasible to deliver, and associated with increases in the targetmechanisms of behavior change. This is the first intervention to use meaning as a behavior change strategy in a PA intervention.Future research will test the efficacy of the intervention in increasing PA compared to a control condition