The study purpose was to test the effectiveness of the self-regulation eHealth intervention ‘MyPlan1.0.’ to increase physical activity (PA) in recently retired Belgian adults. This study was a randomized controlled trial with three points of follow-up/modules (baseline to 1-week to 1-month follow-up). In total, 240 recently retired adults (intervention group [IG]: n = 89; control group [CG]: n = 151) completed all three modules. The IG filled in evaluation questionnaires and received ‘MyPlan1.0.’, an intervention focusing on both pre- and post-intentional processes for behavioural change. The CG only filled in evaluation questionnaires. Self-reported PA was assessed using the long International Physical Activity Questionnaire, usual week version. Repeated-measures multivariate analysis of variances were conducted in SPSS 22.0. On the short-term (baseline to 1 week), the intervention significantly increased walking for transport (IG: +11 min/week, CG: −6 min/week; P < 0.01). On the intermediate-term (baseline to 1 month), the intervention increased transport-related walking (IG: +14 min/week, CG: +6 min/week; P < 0.01), leisure-time walking (IG: +26 min/week, CG: −14 min/week; P < 0.10), leisure-time vigorous PA (IG: +16 min/week, CG: −4 min/week; P < 0.01), moderate-intensity gardening (IG: +4 min/week, CG: −34 min/week; P < 0.10) and voluntary work-related vigorous PA (IG: +28 min/week, CG: +13 min/week; P < 0.10). Results show that our eHealth intervention is effective in recently retired adults. Future studies should include long-term follow-up to examine whether the effects persist over a longer period.