Background <p>Atherosclerotic dyslipidemia has been linked to an increased risk of type 2 diabetes (T2D). While previous studies have established an association between the atherogenic index of plasma (AIP) and insulin resistance, there is a notable lack of large-scale cohort studies examining the relationship between AIP and the risk of T2D in the general population. Therefore, the present longitudinal study aims to examine the association between AIP and the risk of T2D in a cohort of Japanese adults.</p> Methods <p>This retrospective cohort study included a total of 15,453 adults. To evaluate the association between the AIP and the risk of developing T2D, hazard ratios (HR) and 95% confidence intervals (CI) were calculated using Cox proportional hazards regression models. Furthermore, potential nonlinear relationships were explored through Cox proportional hazards regression combined with smooth curve fitting techniques.</p> Results <p>During a mean follow-up period of 6.05 years, 373 adults developed T2D. Elevated AIP was independently associated with a significantly higher risk of T2D after adjusting for potential confounding factors (HR: 2.23, 95% CI: 1.55–3.20, <i>P</i> &lt; 0.0001). Furthermore, a J-shaped relationship between AIP and the incidence of T2D was observed. When AIP levels were below − 0.45, no statistically significant association was found between AIP and T2D risk (HR: 0.35, 95% CI: 0.06–1.95, <i>P</i> = 0.2298). In contrast, AIP levels exceeding this threshold were positively associated with an increased risk of T2D (HR: 2.61, 95% CI: 1.77–3.85, <i>P</i> &lt; 0.0001).</p> Conclusion <p>The AIP demonstrates a J-shaped relationship with the risk of developing T2D in Japanese adults. Consequently, maintaining AIP levels below the identified threshold (-0.45) may help reduce their chances of developing diabetes.</p> Clinical trial number <p>Not applicable.</p>