Background <p>Hypertension is a major public health problem. Despite existing treatment options, overall blood pressure control is still insufficient. Digital health interventions have the potential to improve treatment success. We developed <i>liebria</i>, an internet-based digital intervention accessible via smartphones and computers, designed to support the self-management of hypertension.</p> Methods <p>We tested the effectiveness of <i>liebria</i> in reducing systolic blood pressure and improving other relevant endpoints in adults with hypertension using a parallel randomized controlled trial design. Adults with hypertension (mean age 54.5 years, 47.1% male) were recruited via a German health insurance provider and randomized (1:1; concealed allocation; no blinding of participants) to receive <i>liebria</i> in addition to treatment as usual (<i>n</i> = 52), or treatment as usual alone (<i>n</i>=50). Primary outcome was systolic blood pressure after 3 months (T1). Secondary endpoints were diastolic blood pressure, pulse pressure, quality of life, medication adherence, and social and work-related functioning.</p> Results <p>The study's statistical power was limited due to a smaller sample size (<i>N</i> = 102) than the a priori target sample size (<i>N</i>=676). Intention-to-treat analyses showed an effect of <i>liebria</i> on systolic blood pressure (baseline-adjusted between-group difference -3.5 mmHg, 95% CI -7.13 to 0.13, <i>p</i> = .053, Cohen’s <i>d</i> = 0.36). More participants in the intervention group (78.2% vs. 54.3% in the control group) showed reductions in systolic blood pressure (<i>p</i> = .076). Moreover, <i>liebria</i> had effects on social and work-related functioning. No effects emerged for diastolic blood pressure, pulse pressure, quality of life, or medication adherence. No adverse events or device effects were observed.</p> Conclusions <p>The present study shows first promising results of <i>liebria</i>’s effects on systolic blood pressure and social and work-related functioning. Future studies should aim to replicate effects in a larger sample to increase statistical power.</p> Trial registration <p>German Clinical Trials Register (DRKS00025871); <a href="https://drks.de/search/en/trial/DRKS00025871">https://drks.de/search/en/trial/DRKS00025871</a>; date of registration: October 5, 2021.</p>