Background: Efficient obesity treatment protocols are lacking. This study reports treatment results from a web-based application, originally developed for use in an in-person healthcare setting providing health, overweight, and obesity management. Methods: The web application DrHolmApp (WADHA) was evaluated in adult users two years after it was launched. The WADHA provides a personal and tailored treatment plan comprising a series of detailed action advices on everyday life, constructed from the user’s input to a thorough online questionnaire. Throughout the subscription period, the WADHA users have full access to online healthcare professional support. We conducted a longitudinal cohort study using self-reported data. Results: This study included 940 adult WADHA users (861 female). The median body mass index (BMI) change across all WADHA users was –0.63 BMI points (95% CI: –0.7 to –0.57, P<0.001). 665 (71%) of all WADHA users reduced their BMI (median reduction: 0.94, 95% CI: 0.88 to 1.02). In the subset with obesity (n=675), BMI was reduced in 72%. The median number of days per week with physical activity for at least one hour per day increased with 1.5 days per week (from 2 days per week at baseline, P<0.001). Subsequently, the WADHA users improved their mood, quality of life, and body image satisfaction and reduced their appetite, bullying, and wish for weight loss (all P<0.001). A higher number of consultations associated with greater weight loss (P<0.001) independent of age and degree of obesity at treatment initiation. Conclusions: Seventy-one percent of the WADHA users experienced weight loss, concomitant to an increased level of physical activity, improved mood, quality of life, and body image satisfaction, and reduced appetite, degree of bullying, and wish for weight loss.