This explorative, longitudinal study evaluated the effect of the daily use of a mobile phone-based self-management support system for hypertension in reducing blood pressure (BP) among 50 primary care patients with hypertension over 8 weeks. The self-management system comprises modules for (1) self-reports of BP, pulse, lifestyle, symptoms, and well-being; (2) delivery of reminders and encouragements; and (3) graphical feedback of self-reports. Daily use of the support system significantly reduced BP (systolic BP -7 mm Hg, diastolic BP -4.9 mm Hg) between baseline and week 8, with daily improvements leveling off as the study progressed. Three homogenous subsets of patients were identified who, despite different initial BP levels, showed similar decreases in BP during the study, indicating that patients benefited irrespective of baseline BP. In showing significant reductions in BP, our results suggest that the self-management support system may be a useful tool in clinical practice to help patients self-manage their hypertension.