Aims and background <p>Relative fat mass (RFM) is strongly associated with type 2 diabetes (T2DM) and has been shown to be a better predictor than body mass index (BMI) and waist circumference (WC). This study aims to investigate the association between RFM and incident T2DM among adults in the Tehran Lipid and Glucose Study cohort.</p> Methods <p>Data from 8419 participants (4716 women; mean age, 40.52 years) were analyzed. Logistic regression was used to assess the association of RFM with insulin resistance, metabolic syndrome, and its components at baseline. Cox proportional hazards models were applied to evaluate the association between RFM and incident T2DM. Wald tests were used to compare RFM hazard ratios (HRs) with those of BMI, WC, and waist-to-hip ratio (WHR) in the Cox model.</p> Results <p>Over a median follow-up of 14 years, 1382 individuals (16.42%) developed T2DM. RFM was significantly associated with insulin resistance, metabolic syndrome, and its components at baseline in both men and women. In the multivariable-adjusted model, each standard deviation increase in RFM was associated with a 1.26-fold (1.02–1.57, <i>P</i> = 0.037) higher risk of T2DM in men and a 1.52-fold (1.27–1.81, <i>P</i> &lt; 0.001) higher risk in women. These associations were stronger in younger individuals and remained significant only in women after adjusting for insulin resistance and restricting the analysis to normoglycemic individuals. The increased T2DM risk associated with RFM was comparable to that associated with WC in both men and women (P_difference &gt; 0.05).</p> Conclusions <p>While RFM is strongly associated with the risk of T2DM, particularly in women and younger individuals, it does not outperform WC.</p> <p></p>