Background <p>The complex relationship between obesity and dyslipidemia in type 2 diabetes mellitus (T2DM) remains an area of interest but is not fully understood. This study aimed to evaluate the intricate links between atherogenic index of plasma (AIP), body mass index (BMI), and T2DM risk.</p> Methods <p>Based on data from the China Health and Retirement Longitudinal Study, this analysis comprised 6754 individuals aged 45 and over, free of T2DM in 2011. BMI and AIP were the exposures, with T2DM incidence as the primary focus. Logistic regression models generated odds ratios (ORs), and a thorough decomposition of BMI's impact on T2DM revealed natural indirect and direct effects. The study also examined the complex interactions and combined effects of these two exposures.</p> Results <p>By the end of 2018, 972 individuals were diagnosed with T2DM. The AIP played a significant association in the relationship between BMI and T2DM, explaining 21.7% and 18.9% of the association in different BMI ranges. A significant additive effect was observed between BMI and AIP, with a relative excess risk due to interaction of 0.62. BMI ≥ 24.0 kg/m² and AIP above the median together conferred the highest risk of T2DM, with an OR of 2.31 and a 95% confidence interval (CI) of 1.92–2.79.</p> Conclusion <p>Exposure to overweight/obesity or high AIP raises T2DM risk among Chinese ≥ 45 years, AIP partly mediates BMI-T2DM link.</p>