IMPORTANCE: Lifestyle interventions are effective in diabetes management, with smartphone apps that manage health data and dietary and exercise schedules gaining popularity. However, limited evidence from randomized clinical trials exists regarding the effectiveness of smartphone-based interventions among Asian adults with type 2 diabetes. OBJECTIVE: To compare the effects of a culturally contextualized smartphone-based intervention with usual care on weight and metabolic outcomes. DESIGN, SETTING, AND PARTICIPANTS: This randomized clinical trial conducted at multiple primary care centers in Singapore included 305 adults with type 2 diabetes and body mass index (BMI) of 23 or greater who had literacy in English and smartphone access. Participants were recruited between October 3, 2017, and September 9, 2019, and were randomly assigned (1:1; stratified by gender, age, and BMI) to intervention (99 participants) or control (105 participants) groups. Participants' data were analyzed using intention-to-treat analysis. INTERVENTIONS: Both control and intervention participants received diet and physical activity advice from a dietitian at a baseline face-to-face visit. Intervention participants additionally used a smartphone app to track weight, diet, physical activity, and blood glucose and then communicated with dietitians for 6 months. MAIN OUTCOMES AND MEASURES: Primary outcome was change in body weight, while secondary outcomes were changes in hemoglobin A1c (HbA1c), fasting blood glucose, blood pressure, lipids, and diet. Post hoc analyses included glycemic changes in the subgroup with HbA1c levels of 8% or greater and diabetes medication changes. RESULTS: Among the 204 randomized participants (mean [SD] age, 51.2 [9.7] years; 132 [64.7%] men), baseline mean (SD) BMI was 30.6 (4.3). Compared with the control group, intervention participants achieved significantly greater reductions in weight (mean [SD] change, -3.6 [4.7] kg vs -1.2 [3.6] kg) and HbA1c levels (mean [SD] change, -0.7% [1.2] vs -0.3% [1.0]), with a greater proportion having a reduction in diabetes medications (17 participants [23.3%] vs 4 participants [5.4%]) at 6 months. The intervention led to a greater HbA1c reduction among participants with HbA1c levels of 8% or higher (mean [SD] change, -1.8% [1.4] vs -1.0% [1.4]; P = .001). Intergroup differences favoring the intervention were also noted for fasting blood glucose, diastolic blood pressure, and dietary changes. CONCLUSIONS AND RELEVANCE: In this study, a smartphone-based lifestyle intervention was more effective in achieving weight and glycemic reductions among Asian adults with type 2 diabetes compared with usual care, supporting the use of apps in lifestyle intervention delivery. TRIAL REGISTRATION: anzctr.org.au Identifier: ACTRN12617001112358.