BACKGROUND: Serum symmetric dimethylarginine (SDMA) concentrations are considered a biomarker for renal dysfunction in dogs and humans with acute kidney injury (AKI). No studies have assessed SDMA in cats with AKI. HYPOTHESIS/OBJECTIVES: SDMA correctly identifies cats with azotemic AKI. ANIMALS: Fifteen control cats, 22 with novel AKI, 13 with acute on chronic-AKI (AoC) and 19 with chronic kidney disease (CKD). METHODS: Retrospective study. Cats with azotemia (serum creatinine concentrations >1.7 mg/dL) were defined as having AKI or CKD based on history, clinical signs, clinicopathological findings and diagnostic imaging, and classified using the International Renal Interest Society (IRIS) grading/staging systems. Serum SDMA concentrations were compared between groups with nonparametric methods, and correlations assessed using Spearman's correlation coefficient. Data are presented as median [range]. RESULTS: SDMA concentrations were 11 (8-21) μg/dL, 36 (9-170)μg/dL, 33 (22-75) μg/dL and 25 (12-69) μg/dL in control, novel AKI, AoC and CKD cats. SDMA concentrations were significantly higher in cats with novel AKI (P < .001), AoC (P < .001) and CKD (P < .01) compared to controls. SDMA concentrations were significantly higher in cats with more advanced AKI (IRIS grade IV-V) compared to less severe AKI (IRIS grade II). Serum creatinine and SDMA concentrations had a significant correlation in cats with novel AKI (rs  = 0.826, n = 22; P < .001) and a significant correlation when all cats across all 4 groups were considered together (rs  = 0.837, n = 69; P < .001). CONCLUSIONS AND CLINICAL IMPORTANCE: Serum SDMA concentrations are elevated in cats with established AKI (novel and AoC) and CKD, providing evidence for use of SDMA as a biomarker for AKI in cats.