Background/Objectives: Post-translationally modified peptide fragments of fetuin-A (FetA) were identified as a potential biomarker of diabetic kidney disease (DKD). An independent association between urinary FetA-derived peptide levels (uPTM3-FetA) and DKD progression in patients with type 2 diabetes (T2D) was evidenced. This study aimed to explore uPTM3-FetA excretion and its associations with insulin resistance, inflammatory and metabolic biomarkers in patients with type 1 diabetes (T1D), and the normal albuminuria and estimated glomerular filtration rate (eGFR) > 60 mL/min/1.73 m². Methods: uPTM3-FetA levels in aliquots of 24 h urine specimens, routine laboratory renal, metabolic and inflammatory tests, adipokines (leptin, adiponectin, resistin), and insulin resistance, assessed as the estimated glucose disposal rate (eGDR), were measured in a cohort of 169 adult T1D patients. To evaluate the changes in early renal dysfunction, the cohort was divided according to the median eGFR. Above- and below-median-eGFR groups were considered as having normal and declining kidney function, respectively. Results: The median (IQR) uPTM3-FetA level was 11.7 (8.43–16.65 µg/24 h), with no significant difference between males and females, as well as normal and declining kidney function patients. However, a sex-specific analysis revealed a significantly higher uPTM3-FetA excretion in male T1D patients with lower eGFRs, when compared to those with higher eGFRs, whereas no such difference was observed in female patients. BMI, hs-CRP, resistin and HDL-cholesterol were identified as independent predictors of uPTM3-FetA excretion. Conclusions: Our results implicate the potential role of uPTM3-FetA in the detection of an early renal dysfunction in male patients with T1DM and pinpoint the importance of a sex-specific approach in diabetes diagnostics and research.