Physical activity (PA) plays an important role in healthy aging among older adults. Web-based interventions may present a novel and accessible way to promote PA in older adults. This study systematically explored the effectiveness of web-based interventions designed to promote PA in older adults. A systematic search of published and unpublished literature using PubMed, ProQuest, and ProQuest Dissertations and Theses identified 10 qualifying studies. From these, we identified three methods of web-based delivery for PA intervention in older adult populations: (a) online assessment; (b) PA online materials; and (c) tailored messages. Seven of the10 studies also reported significant changes in PA levels between the intervention and control groups over their respective study periods. Results also indicated that individually tailoring web-based interventions may increase the effectiveness of the intervention. Results of this systematic review will help practitioners develop more effective web-based interventions targeting PA among older adults.