IMPORTANCE: The association between an overall healthy lifestyle and the subsequent risk of microvascular complications among patients with diabetes remains unclear. OBJECTIVE: To examine the association between adherence to a healthy lifestyle before and after diabetes diagnosis and the risk of subsequent microvascular complications among adults with diabetes. DESIGN, SETTING, AND PARTICIPANTS: This prospective cohort study included incident patients with type 2 diabetes who were free of cardiovascular disease and cancer at the time of diabetes diagnosis and completed the diabetes supplementary questionnaires in the Nurses' Health Study (in 2000 and 2005) and the Health Professionals Follow-Up Study (in 2000, 2004, and 2008) in the US. Data were analyzed from April to August 2021. EXPOSURES: Diet and lifestyle factors before and after diabetes diagnosis were assessed by validated questionnaires. A healthy lifestyle consisted of nonsmoking, having a healthy body weight (a body mass index of ≥18.5 or <25), engaging in moderate-to-vigorous physical activity (≥150 minutes per week), consuming a high-quality diet (top 40th percentile of the Alternative Healthy Eating Index), and moderate alcohol drinking (5-15 g/d for women and 5-30 g/d for men). MAIN OUTCOMES AND MEASURES: Physician-diagnosed microvascular complications including diabetic neuropathy, retinopathy, nephropathy, and foot disorders were self-reported at questionnaire surveys. RESULTS: A total of 7077 patients with type 2 diabetes were included in the cohort (4982 women in NHS and 2095 men in HPFS, mean [SD] age 61 [8.8], 94.2% White). During follow-up, 2878 patients developed microvascular complications. After multivariable adjustment, adherence to a healthy lifestyle before and after diabetes diagnosis were both associated with a lower risk of developing microvascular complications. The relative risk (RR) for participants with 4 or more low-risk lifestyle factors before diabetes diagnosis compared with zero was 0.73 (95% CI, 0.60-0.91) for any microvascular complications, 0.71 (95% CI, 0.54-0.93) for diabetic neuropathy, 0.76 (95% CI, 0.57-1.01) for diabetic retinopathy, 0.42 (95% CI, 0.23-0.79) for diabetic nephropathy, and 0.60 (95% CI, 0.35-1.00) for diabetic foot disorders. Similar results were observed for adherence to a healthy lifestyle after diabetes diagnosis, with an RR of 0.68 (95% CI, 0.55-0.83) for any microvascular complications, 0.67 (95% CI, 0.51-0.88) for diabetic neuropathy, 0.65 (95% CI, 0.48-0.86) for diabetic retinopathy, 0.57 (95% CI, 0.34-0.98) for diabetic nephropathy, and 0.62 (95% CI, 0.37-1.05) for diabetic foot disorders. In addition, greater improvement in lifestyle factors from before to after diabetes diagnosis was also significantly associated with a lower risk of neuropathy or total microvascular complications. Each increment in number of low-risk lifestyle factors was associated with a 6% (RR, 0.94; 95% CI, 0.90-0.98) lower risk for any microvascular complications and a 9% (RR, 0.91; 95% CI, 0.86-0.96) lower risk for diabetic neuropathy. Consistent results were observed when analyses were stratified by age at diabetes diagnosis, sex/cohort, or lifestyle factors before diabetes diagnosis. CONCLUSIONS AND RELEVANCE: In this cohort study, adhering to an overall healthy lifestyle was associated with a significantly lower risk of microvascular complications among individuals with diabetes. These findings suggest substantial reduction in the burden of microvascular complications associated with adopting a healthy lifestyle among patients with type 2 diabetes.