Despite advances in type 2 diabetes (T2D) management, unmet needs remain for therapies that effectively control hyperglycaemia while addressing the comorbid metabolic disorders1, 2. Here we assessed the efficacy and safety of the dual glucagon receptor (GCGR)/glucagon-like peptide-1 receptor (GLP-1R) agonist mazdutide monotherapy versus placebo in Chinese adults with T2D inadequately controlled with diet and exercise alone. In this phase 3 trial, 320 participants (mean HbA 1c of 8.24%, BMI of 28.2 kg/m2, and diabetes duration of 1.9 years) were randomised 1:1:1 to receive weekly subcutaneous injections of mazdutide 4 mg, 6 mg, or placebo for 24 weeks, followed by a 24-week extended mazdutide treatment. At week 24, mazdutide significantly reduced HbA 1c versus placebo (primary endpoint): -1.57% with mazdutide 4 mg and -2.15% with mazdutide 6 mg, versus -0.14% with placebo, with treatment differences of -1.43% and -2.02% (both p <0.0001). Significant weight loss at week 24 occurred with -5.61% (4 mg) and -7.81% (6 mg) versus -1.26% (placebo) (both p<0.0001). Additionally, significantly more participants with mazdutide achieved clinically relevant HbA 1c target (<7.0%), weight loss goal (≥5%), and composite endpoints (HbA 1c <7.0% and weight loss ≥5%) versus placebo (all p <0.0001). The most common adverse events—diarrhoea, decreased appetite, and nausea—were consistent with GLP-1R agonists. These results establish mazdutide monotherapy as an effective intervention providing clinically meaningful glycaemic control and weight reduction alongside a favourable safety profile in this population.