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Background/Objectives: The clinical profile of serum Maresin-1 (MaR1) in relation to diabetic retinopathy phenotypes and proteinuria in type 2 diabetes mellitus (T2DM) remains unclear. We evaluated serum MaR1 across healthy controls and patients with T2DM without diabetic retinopathy (DR), non-proliferative DR (NPDR), or proliferative DR (PDR), and examined the relationship between MaR1 and the urine protein-to-creatinine ratio (UPCR). Methods: This single-center cross-sectional study included 93 participants. Serum MaR1 was measured by ELISA. Group differences were assessed with the Kruskal-Wallis test and Holm-adjusted post hoc tests. DR phenotypes were analyzed among patients with T2DM. The MaR1-UPCR relationship was examined using correlation and adjusted regression models. Results: MaR1 differed across groups (H = 49.36, p < 0.001, epsilon² = 0.521). The median MaR1 was 89.8 (82.8-97.2) pg/mL in controls and 34.3 (33.0-36.0), 35.8 (34.4-36.7), and 34.0 (33.1-35.7) pg/mL in T2DM without DR, NPDR, and PDR, respectively. MaR1 was higher in controls than in all T2DM groups, whereas T2DM groups did not differ. Within T2DM, MaR1 was not associated with DR stage (H = 4.44, p = 0.109; rho = -0.001, p = 0.996). MaR1 was inversely related to the UPCR overall (rho = -0.272, p = 0.008), but not within T2DM (rho = -0.057, p = 0.634) or in adjusted models. Conclusions: MaR1 was markedly lower in T2DM than in controls. This reduction was not explained by DR stage or proteinuria. These findings indicate that MaR1 should be interpreted as a T2DM-associated systemic alteration rather than as a marker of retinopathy stage or proteinuria.