AIMS: There is limited research regarding insulin dosing changes following adoption of plant-based diets. We conducted a nonrandomized crossover trial utilizing two plant-based diets (Dietary Approaches to Stop Hypertension, or DASH, and Whole Food, Plant-Based, or WFPB) to assess acute changes in insulin requirements and associated markers among individuals with insulin-treated type 2 diabetes. METHODS: Participants (n = 15) enrolled in a 4-week trial with sequential, one-week phases: Baseline, DASH 1, WFPB, and DASH 2. Each diet was ad libitum and meals were provided. RESULTS: Compared to baseline, daily insulin usage was 24%, 39%, and 30% lower after DASH 1, WFPB, and DASH 2 weeks respectively (all p < 0.01). Insulin resistance (HOMA-IR) was 49% lower (p < 0.01) and the insulin sensitivity index was 38% higher (p < 0.01) at the end of the WFPB week before regressing toward baseline during DASH 2. Total, LDL, and HDL cholesterol, leptin, urinary glucose, and hsCRP decreased to a nadir at the end of the WFPB week before increasing during DASH 2. CONCLUSIONS: Adopting a DASH or WFPB diet can result in significant, rapid changes in insulin requirements, insulin sensitivity, and related markers among individuals with insulin-treated type 2 diabetes, with larger dietary changes producing larger benefits.