The study analyzed data from 23,722 participants (55% female) with a mean age of 59.1 years from the EPIC-Norfolk prospective cohort study in Great Britain. Adherence to the Planetary Health Diet (PHD) was assessed based on 13 food groups and two nutrients with a score range of 0–140 points using food frequency questionnaires at three time points. During a mean follow-up of 19.4 years, there were 3,496 cases of type 2 diabetes (T2D). Higher adherence to PHD was associated with lower incidence of T2D; comparing the highest quintile (85.7–117.8 points) with the lowest (33.9–68.4 points), the hazard ratio (HR) was 0.68 (95% CI: 0.61–0.76) in the most adjusted model. The estimated population attributable fraction for T2D at adherence below the 80th percentile (85.7 points) was 12.3% (95% CI: 9.2–15.3%). Participants in the highest PHD quintile had 18.4% lower GHG emissions (95% CI: −19.3% to −17.5%) compared to the lowest quintile. The main limitation is the possibility of residual confounding due to the observational design. The findings support the promotion of PHD for T2D prevention and planetary sustainability.