Research indicates that adolescents with type 1 diabetes mellitus (T1DM) may develop both metabolic syndrome (MetS) and oxidative stress. The purpose of this study was to test the hypothesis that MetS could potentially affect antioxidant defense parameters. The study recruited adolescents aged 10–17 who had been diagnosed with T1DM, and divided them into two groups: “MetS+” (n = 22), who had been diagnosed with MetS, and “MetS−” (n = 81), who did not have metabolic syndrome. A control group consisting of 60 healthy peers without T1DM was included for comparison. The study examined cardiovascular parameters, such as complete lipid profile and estimated glucose disposal rate (eGDR), as well as markers of antioxidant defense. The results revealed a statistically significant difference between the MetS+ and the MetS− group in terms of total antioxidant status (TAS) (1.186 mmol/L vs. 1.330 mmol/L), and oxidative stress index (OSI) levels (0.666 vs. 0.533). Furthermore, multivariate correspondence analysis identified individuals with HbA1c < 8%; eGDR > 8 mg/kg/min, using either flash or continuous glucose monitoring systems, as MetS− patients. The study also found that eGDR (AUC 0.85,p< 0.001), OSI and HbA1c (AUC 0.71,p< 0.001) markers may be useful for diagnosing the onset of MetS in adolescents with T1DM.