This study provides insight in the effectiveness of a print-delivered and a Web-based physical activity (PA) intervention (with or without additional environmental information on local PA possibilities) among people aged over 50. Intervention groups (print-delivered basic [PB; n = 439], print-delivered environmental [PE; n = 435], Web-based basic [WB; n = 423], Web-based environmental [WE; n = 432]) and a control group (n = 411) were studied in a clustered randomized controlled trial. Participants received three times tailored advice within four months, targeting the psychosocial determinants of PA, and additional environmental information (in two conditions). Outcome measures include weekly minutes and days of sufficient PA 6 months after the start, considering age, gender, educational level, body mass index, the presence of a chronic physical limitation and PA intentions as possible effect moderators. The results showed that the PB (B = 192.47; 95% CI = 75.24-309.71; P = 0.003), the PE (B = 229.31; 95% CI = 108.73-349.89; P = 0.001) and the WB-intervention condition (B = 214.25; 95% CI 88.65-339.85; P = 0.002) resulted in significantly increased weekly minutes of PA. Only the printed conditions resulted in increased days of sufficient PA (PB: B = 0.63; 95% CI = 0.35-0.92; P < 0.001; PE: B = 0.55; 95% CI = 0.26-0.85; P = 0.001). Additional environmental information did not increase intervention effects. Differences in effect were found between age and gender subgroups. In conclusion, both printed and Web-based interventions can be effective in increasing PA in adults aged over 50.