We evaluated the associations between changes in plant-based diets and subsequent risk of type 2 diabetes.We prospectively followed 76,530 women in the Nurses' Health Study (NHS) (1986-2012), 81,569 women in NHS II (1991-2017), and 34,468 men in the Health Professionals Follow-up Study (1986-2016). Adherence to plant-based diets was assessed every 4 years with the overall plant-based diet index (PDI), healthful PDI (hPDI), and unhealthful PDI (uPDI). We used multivariable Cox proportional hazards models to estimate hazard ratios (HRs). We pooled results of the three cohorts using meta-analysis.We documented 12,627 cases of type 2 diabetes during 2,955,350 person-years of follow-up. After adjustment for initial BMI and initial and 4-year changes in alcohol intake, smoking, physical activity, and other factors, compared with participants whose indices remained relatively stable (±3%), participants with the largest decrease (>10%) in PDI and hPDI over 4 years had a 12-23% higher diabetes risk in the subsequent 4 years (pooled HR, PDI 1.12 [95% CI 1.05, 1.20], hPDI 1.23 [1.16, 1.31]). Each 10% increment in PDI and hPDI over 4 years was associated with a 7-9% lower risk (PDI 0.93 [0.91, 0.95], hPDI 0.91 [0.87, 0.95]). Changes in uPDI were not associated with diabetes risk. Weight changes accounted for 6.0-35.6% of the associations between changes in PDI and hPDI and diabetes risk.Improving adherence to overall and healthful plant-based diets was associated with a lower risk of type 2 diabetes, whereas decreased adherence to such diets was associated with a higher risk.