Insulin resistance (IR) is a major contributor to type 2 diabetes, cardiovascular disorders, and metabolic syndrome. Although often associated with obesity, increasing evidence shows that individuals with normal body mass index (BMI) may also develop IR, especially within Asian populations where a “thin-fat” phenotype is common. This cross-sectional study was conducted at a tertiary care hospital in Chennai to evaluate the prevalence and distribution of IR among normal-BMI adults aged 18–50 years. Using the Homeostatic Model Assessment of Insulin Resistance (HOMA-IR), 67.2% of participants were found to have varying degrees of IR, despite their BMI being within the normal range. No significant associations were observed with age or gender, but fasting blood glucose showed a strong correlation with IR. The findings emphasize that BMI alone is insufficient for screening and that early detection of IR using markers such as HOMA-IR and fasting glucose may help in preventing long-term metabolic complications.