We report a case of a 47-year-old female patient with ceftriaxone (CTRX)-associated pseudolithiasis. CTRX was administered at a dosage of 2g/day for 8 days because of colonic diverticulitis. A routine abdominal computed tomography (CT) scan was performed to investigate the diverticulitis. However, the CT scan demonstrated stones and sludge in the gallbladder, which had not been present before CTRX administration. Therefore, we diagnosed the patient with pseudolithiasis caused by CTRX and stopped CTRX administration. The stones and sludge disappeared 6 days after stopping CTRX administration. This underreported adverse effect of CTRX should be considered when treating both children and adult patients.