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Background/Objectives: Insulin resistance is increasingly recognized as a cause of colorectal neoplasms, but its measurement requires fasting insulin, which is not routinely available in clinical settings. The triglyceride-glucose (TyG) index, derived from fasting triglyceride and glucose, has emerged as a simple surrogate of insulin resistance. We aimed to evaluate the association of the TyG index with colorectal adenoma, identify a clinically usable cut-off, and examine whether the association is preserved across major subgroups. Methods: We conducted a cross-sectional analysis of 7251 asymptomatic adults who underwent screening colonoscopy and same-day biochemistry at university hospital health care centers in Daejeon and Chungcheong province of South Korea between November 2019 and June 2022. The TyG index was calculated as ln[fasting triglycerides (mg/dL) × fasting glucose (mg/dL)/2]. Multivariable logistic regression was used to estimate odds ratios (ORs) for adenoma; discrimination was evaluated by area under the receiver-operating-characteristic curve (AUC), and the optimal cut-off was identified by Youden's J. Large adenoma (≥10 mm) was analyzed as a secondary outcome. Results: Among 7251 participants (mean age 54.1 ± 11.2 years; 59.7% male; mean BMI 24.7 ± 3.4 kg/m²), 2402 (33.1%) had at least one colorectal adenoma. Adenoma prevalence rose monotonically across TyG quartiles (Q) (Q1, 26.3%; Q2, 32.0%; Q3, 35.5%; Q4, 38.7%; p for trend <0.001). A 1-standard deviation (SD) increase in TyG index was associated with adenoma prevalence in the fully adjusted model (OR 1.13, 95% confidence interval (CI) 1.06-1.20), and the Q4-versus-Q1 OR was 1.29 (1.09-1.53). The optimal cut-off for adenoma was TyG index = 8.55 (AUC 0.564, sensitivity 59.1%, specificity 50.8%); the association was modestly stronger for large adenoma (AUC 0.585; adjusted OR per 1-SD 1.25, 1.09-1.43). Subgroup analyses showed consistent effects across sex, age, body mass index, hypertension, diabetes, and metabolic-syndrome strata (all p for interaction >0.17). Conclusions: In a large screening cohort, an elevated TyG index was associated with the presence of colorectal adenoma, with a graded dose-response relationship and a modestly more pronounced association for large adenoma. Although discrimination by TyG index alone is too modest to support its use as a stand-alone screening tool, the index may serve as a low-cost adjunct within a multifactorial risk-stratification framework.