Key Clinical MessageClinicians should be mindful of the rare occurrence of spontaneous bacterial peritonitis in essential thrombocythemia with extensive splanchnic vein thrombosis, especially when patients with ascites exhibit fever and abdominal pain.AbstractSpontaneous bacterial peritonitis (SBP) complicating extensive splanchnic vein thrombosis (SVT) is a rare manifestation of essential thrombocythemia (ET). In the absence of any hypercoagulable state, JAK2 mutation can be an important risk factor for extensive SVT. Evaluation for SBP is crucial when non‐cirrhotic patient exhibits fever, abdominal pain and tenderness in the background of ascites after ruling out common pathologies such as tubercular peritonitis, acute pancreatitis, Budd‐Chiari syndrome and ovarian malignancy. We present a case of SBP complicating pre‐hepatic portal hypertension with ascites in a 44‐years‐old female. On further evaluation, extensive SVT with portal cavernoma in the setting of ET was identified. She was managed with cytoreductive therapy and anticoagulation, resulting in symptom resolution.