Hydronephrosis is an uncommon condition in cats and dogs, characterized by progressive dilation of the renal pelvis and subsequent atrophy of the renal parenchyma. This report describes an 8-year-old spayed female domestic longhair cat diagnosed with left-sided hydronephrosis and concurrent polycystic kidney disease (PKD) after comprehensive clinical, laboratory, and imaging evaluation. Because the contralateral kidney appeared functionally preserved, unilateral nephrectomy was selected as the most appropriate treatment option. During surgical manipulation, however, the affected kidney unexpectedly separated completely from its attachments before vascular ligation could be performed, without evidence of hemorrhage. The procedure was completed without complication, and no intraoperative or postoperative adverse events were recorded. The patient recovered uneventfully and remained stable throughout the perioperative period. To the authors’ knowledge, this represents the first documented case in veterinary medicine of spontaneous renal autoavulsion occurring during nephrectomy. This unusual finding underscores the severe structural fragility that can result from chronic obstructive renal disease and advanced parenchymal destruction. In addition, it suggests that renal autoavulsion may represent a previously unrecognized end-stage manifestation of longstanding hydronephrosis, particularly when associated with concurrent renal pathology such as PKD. This case emphasizes the importance of thorough preoperative assessment, careful surgical planning, and awareness of potentially extreme anatomic alterations in chronically diseased kidneys