Objective To investigate the relationship between serum legumain (LGMN) levels and carotid atherosclerosis (CAS) in patients with type 2 diabetes mellitus (T2DM).Methods We enrolled 111 T2DM patients from Baoding No.1 Central Hospital between May 2023 and May 2024, classifying them into T2DM group (n = 55) and T2DM+CAS group (n = 56) based on carotid intima-media thickness (CIMT), with 55 healthy controls (NC group). Clinical data were collected, serum LGMN was measured by ELISA, and correlations were analyzed using Spearman's method. Multivariate logistic regression identified risk factors for T2DM+CAS, and receiver operating characteristic (ROC) curves evaluated LGMN's diagnostic value.Results Significant differences existed among groups for BMI, SBP, FBG, HbA1c, FINS, HOMA-IR, and LGMN (all P < 0.05). T2DM+CAS group showed highest HbA1c and LGMN levels (P < 0.05 vs NC). LGMN positively correlated with BMI (r? =0.303), SBP (r? =0.400), FPG (r? =0.639), HbA1c (r? =0.699), FINS (r? =0.340), HOMA-IR (r? =0.493), and CIMT (r? =0.580) (all P = 0.000). Elevated HbA1c [O^R = 1.944 （95% CI: 1.311, 2.883）] and LGMN [O^R = 1.495 （95% CI: 1.273, 1.756）] were independent risk factors for T2DM+CAS (P < 0.05). LGMN's AUC for diagnosing T2DM+CAS was 0.870 (95% CI: 0.819, 0.924), with 87.5% (95% CI: 0.788, 0.962) sensitivity and 73.6% (95% CI: 0.654, 0.819) specificity.Conclusions Serum LGMN elevation is significantly associated with CAS in T2DM patients, potentially serving as a novel biomarker for T2DM complicated by CAS.