To investigate safety and effectiveness of velagliflozin oral solution as sole therapy in naïve and previously insulin-treated diabetic cats. 252 client-owned cats receiving ≥ 2 doses of velagliflozin; 214 (85%) naïve diabetics and 38 (15%) insulin-treated diabetics. Prospective, baseline-controlled, open-label clinical field trial. Cats received velagliflozin orally, once daily. Physical examinations and blood collections were performed days 0, 3, 7, 30, 60, 120, and 180. Data are median (range). Screening blood glucose (BG) was 436 mg/dL (272 to 676 mg/dL). On days 30, 60, 120, and 180, single BG after receiving velagliflozin was 153 mg/dL (62 to 480 mg/dL), 134 mg/dL (64 to 414 mg/dL), 128 mg/dL (55 to 461 mg/dL), and 125 mg/dL (77 to 384 mg/dL), respectively. Screening fructosamine was 538 µmol/L (375 to 794 µmol/L). On the same recheck days, fructosamine was 310 µmol/L (204 to 609 µmol/L), 286 µmol/L (175 to 531 µmol/L), 269 µmol/L (189 to 575 µmol/L), and 263 µmol/L (203 to 620 µmol/L). At day 180, 81% of 158 cats remaining had BG and/or fructosamine within reference ranges; 88.6% (124 of 140) and 87.7% (121 of 138) showed improvement in polyuria and polydipsia, respectively. Ketonuria developed in 35 cats (13.9%), including 18 (7.1%) that had ketoacidosis. Ketoacidosis was less common in naïve diabetic cats (11 of 214 [5.1%]) compared to insulin-treated diabetic cats (7 of 38 [18.4%]). At ketoacidosis diagnosis, 14 of 18 cats (77.8%) were euglycemic (ie, BG < 250 mg/dL). Most episodes of ketosis or ketoacidosis (30 of 35 [85.7%]) occurred within the first 14 days of treatment. Insulin-treated diabetic cats were less likely to complete the trial. No clinical hypoglycemia occurred. Velagliflozin improved glycemic parameters and clinical signs in diabetic cats. Velagliflozin provides an alternative to insulin as a stand-alone treatment of diabetic cats.