South Asians, who represent one-quarter of the world’s population, are at high risk of type-2 diabetes (T2D). Intensive lifestyle modification (healthy diet and physical activity) is effective at preventing T2D amongst South Asians with impaired glucose tolerance, but this approach is limited by high-cost, poor scalability and low impact on T2D burden. We will complete a cluster-randomised clinical trial at 120 locations across India, Pakistan, Sri Lanka and the UK. We will compare family-based intensive lifestyle modification (22 health promotion sessions from a community health worker, active group, N=60 sites) vs usual care (1 session, control group, N=60 sites) for prevention of T2D, amongst 3,600 non-diabetic South Asian men and women with central obesity (waist≥100cm) and/or prediabetes (HbA1c≥6.0%). Participants will be followed annually for 3 years. The primary endpoint will be new-onset T2D (physician diagnosis on treatment or HbA1c≥6.0%, predicted N~734 over 3 years). Secondary endpoints will include waist and weight in the index case and family members. Our study has 80% power to identify a reduction in T2D risk with family-based intervention vs usual care of: 30% in South Asians with central obesity; 24% in South Asians with prediabetes; and 24% overall. Health economic evaluation will determine cost-effectiveness of family based lifestyle modification for prevention of T2D amongst South Asians with central obesity and / or prediabetes. The impact of gender and socio-economic factors on clinical utility and cost-effectiveness will be investigated.Our results will determine whether screening by waist circumference and/or HbA1c, coupled with intervention by family-based lifestyle modification, is an efficient, effective and equitable strategy for prevention of T2D in South Asians. Our findings will thereby provide a robust evidence base for scalable community-wide approaches to reverse the epidemic of T2D amongst the >1.5 billion South Asians worldwide.