Background <p>Estimated small dense low-density lipoprotein cholesterol (sdLDL-C) was related to atherosclerosis, coronary heart disease and metabolic syndrome. Despite these findings, limited evidence is available on the relationship between sdLDL-C levels and the onset of diabetes mellitus (DM).</p> Methods <p>The study analyzed data from 118,080 adults enrolled at the Rich Healthcare Group between 2010 and 2016. The relationship between sdLDL-C levels and the risk of DM was examined using Cox proportional hazards regression. In order to evaluate potential nonlinear associations, cubic spline functions and smooth curve fitting were incorporated into the Cox regression framework. Furthermore, a two-piecewise Cox proportional hazards regression was employed to pinpoint the inflection point of sdLDL-C regarding DM risk.</p> Results <p>SdLDL-C was found to have a significant correlation with DM risk after controlling for confounders (HR: 1.04, 95% CI: 1.03–1.04, <i>P</i> &lt; 0.0001). The inflection point for sdLDL-C was calculated to be 29.49&#xa0;mg/dL. The HR was measured at 1.08 (95% CI: 1.06–1.10) when sdLDL-C was below 29.49&#xa0;mg/dL, and it decreased to 1.03 (95% CI: 1.03–1.04) when above 29.49&#xa0;mg/dL.</p> Conclusion <p>This investigation reveals a nonlinear positive connection between sdLDL-C levels and the risk of developing DM in Chinese adults. Notably, sdLDL-C levels lower than 29.49&#xa0;mg/dL were strongly associated with a greater risk of DM.</p> Clinical trial number <p>not applicable.</p>