Background: Long-term weight loss in people living with obesity can reduce the risk and progression of noncommunicable diseases. Observational studies suggest that digital coaching can lead to long-term weight loss. Objective: We investigated whether an eHealth lifestyle coaching program for people living with obesity with or without type 2 diabetes led to significant, long-term (12-month) weight loss compared to usual care. Methods: In a randomized controlled trial that took place in 50 municipalities in Denmark, 340 people living with obesity with or without type 2 diabetes were enrolled from April 16, 2018, to April 1, 2019, and randomized via an automated computer algorithm to an intervention (n=200) or a control (n=140) group. Patients were recruited via their general practitioners, the Danish diabetes organization, and social media. The digital coaching intervention consisted of an initial 1-hour face-to-face motivational interview followed by digital coaching using behavioral change techniques enabled by individual live monitoring. The primary outcome was change in body weight from baseline to 12 months. Results: Data were assessed for 200 participants, including 127 from the intervention group and 73 from the control group, who completed 12 months of follow-up. After 12 months, mean body weight and BMI were significantly reduced in both groups but significantly more so in the intervention group than the control group (-4.5 kg, 95% CI -5.6 to -3.4 vs -1.5 kg, 95% CI -2.7 to -0.2, respectively; P<.001; and -1.5 kg/m2, 95% CI -1.9 to -1.2 vs -0.5 kg/m2, 95% CI -0.9 to -0.1, respectively; P<.001). Hemoglobin A1c was significantly reduced in both the intervention (-6.0 mmol/mol, 95% CI -7.7 to -4.3) and control (-4.9 mmol/mol, 95% CI -7.4 to -2.4) groups, without a significant group difference (all P>.46). Conclusions: Compared to usual care, digital lifestyle coaching can induce significant weight loss for people living with obesity, both with and without type 2 diabetes, after 12 months.