A 6-year-old neutered male Poodle with a body weight of 2.7 kg was diagnosed with a congenital extrahepatic portoazygos shunt presented with progressive hepatic encephalopathy. Five days after surgical attenuation, the patient showed post-attenuation neurological signs (PANS) such as generalized tonic-clonic seizure, dull mentation, and tremor. PANS were successfully managed with antiepileptic drugs, but third-space fluid accumulation (ascites and peripheral edema) and phlebectasia were newly identified, suggesting marked portal hypertension (PHT). Telmisartan, spironolactone, carvedilol, and prednisolone were sequentially administered. Three months after surgery, both abnormal clinicopathological values such as anemia and hypoalbuminemia, and clinical signs completely resolved. Herein, we report successful management strategies for PANS and PHT in a dog following surgical attenuation of a congenital portoazygos shunt.