With more social support and environment-centered interventions being recommended in web-based interventions, this study examined the efficacy of three intervention conditions aimed at promoting physical activity (PA) in older adults. The efficacy analyses included the self-reported PA level, stage of change for PA and awareness about PA among participants. Eligible participants (N = 149; M = 65 years old, SD = 6), recruited in a unique Belgian French-speaking municipality, were randomized in four research arms for a 3-month intervention: (i) web-based; (ii) center-based; (iii) mixed (combination of web- and center-based); and (iv) control (no intervention). Web-based condition included a PA website and monthly tailored emails whereas center-based condition comprised 12 sessions (1 per week) of group exercising. With a significant increase in PA, the PA stage of change and the PA awareness at 12 months, the mixed intervention condition seemed to include the key social and motivating elements for sustainable behavior change. Center-based intervention was more likely to produce significant improvements of the PA level and the stage of change for PA change whereas web-based intervention was more likely to extend the awareness about PA.