Objective This study aimed to evaluate the effectiveness of the app‐based, multimodal weight loss program zanadio. Methods A randomized‐controlled trial was conducted from January 2021 to March 2022. A total of 150 adults with obesity were randomized into an intervention group and used zanadio for 1 year or into a wait list control group. The primary end point, weight change, and the secondary end points, quality of life, well‐being, and waist to height ratio, were assessed every 3 months for up to 1 year via telephone interviews and online questionnaires. Results After 12 months, participants of the intervention group lost, on average, −7.75% (95% CI: −9.66% to −5.84%) of their initial weight, achieving a clinically relevant and statistically stronger weight reduction than the control group (mean = 0.00% 95% CI: −1.98% to 1.99%). All secondary end points improved significantly in the intervention group, with significantly greater improvements in well‐being and waist to height ratio than in the control group. Conclusions This study showed that adults with obesity who have used zanadio achieved a significant and clinically relevant weight loss within 12 months and improved further obesity‐related health variables compared with a control group. Because of its effectiveness and flexible applicability, the app‐based multimodal treatment zanadio might alleviate the present care gap for patients with obesity in Germany.