Rationale:The diagnosis of mesenteric ischemia in critically ill patients remains challenging; however, the aquarium sign, comprising a large number of bubble images in the right cardiac chambers on echocardiography, may be used as a point-of-care ultrasound finding to diagnose acute mesenteric ischemia (AMI).Patient concerns:A 65-year-old woman diagnosed with lymphoma was urgently admitted to the intensive care unit with suspected tumor lysis syndrome. High-dose vasopressor and inotropic agents were required to manage the patient’s shock with marked lactic acidosis and peripheral hypoperfusion with mottled skin, and multidisciplinary treatment was initiated. By day 6, the lactate levels normalized and there were no abnormal abdominal findings. An echocardiogram was performed to examine the mass lesion associated with lymphoma in the right atrium and evaluate the hemodynamics; it revealed an “aquarium sign.” Similar findings were found in the inferior vena cava and portal vein.Diagnoses:Contrast-enhanced computed tomography of the abdomen revealed hepatic portal vein gas, poor contrast of the colon wall, and intramural emphysema, and a diagnosis of AMI was made. Lower gastrointestinal endoscopy showed necrosis of the colon.Interventions:The patient underwent urgent subtotal colorectal resection.Outcomes:Although a tracheostomy was required, the patient’s general condition improved after surgery, and she was discharged to the ward without mechanical ventilatory support in the intensive care unit on Day 19.Lessons:In patients with risk factors for AMI, repeated evaluation for the presence of aquarium signs by echocardiography may be warranted, even if there are no abdominal findings or abnormalities in biomarkers, such as lactate levels and trends. When the aquarium sign is found, AMI should be aggressively suspected, and a definitive diagnosis should be made to initiate early therapeutic intervention.