Background/Objectives: Low vitamin D (VD) has been associated with diabetic kidney disease (DKD). Nevertheless, VD normality concentrations remain under discussion. Objectives: To evaluate VD concentrations in early and advanced DKD stages according to the Endocrine Society and Institute of Medicine standards in patients with type 2 diabetes mellitus (T2DM). Methods: A cross-sectional study included 80 patients with T2DM and DKD in early (urine albumin–creatinine ratio (UACR) between 30 and 299 mg/g), group 1 (n = 29), and advanced stage (UACR ≥ 300 mg/g), group 2 (n = 51). Results: Compared to group 1, patients in group 2 showed lower 25(OH)D concentrations (26.7 ± 10.1 vs. 30.2 ± 8.0 ng/mL; p ≤ 0.05). In group 2, when the serum VD concentrations were stratified according to the Endocrine Society, VD concentrations ≥ 30 ng/mL were associated with a considerably reduction in UACR compared with VD < 20 ng/mL (365.9 ± 268.3 vs. 937.6 ± 634 mg/g; p < 0.05), but not when compared to the range of 20–29.9 ng/mL (739.8 ± 1138 vs. 937.6 ± 634 mg/g; p = NS). Additionally, in this group, our regression model showed that each unit increase in VD was associated with a UACR reduction of 26.1 mg/g (R² = 0.103; p < 0.05). Conclusions: Higher vitamin D concentrations were associated with improved UACR only in clinical DKD, particularly when VD was at least 30 ng/mL.