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Crossed fused renal ectopia is a rare, typically asymptomatic congenital malformation of the upper urinary tract. Diagnosis is often incidental during imaging. We present the case of a man in his 50s presenting to the Emergency Department with his first episode of acute abdominal pain, secondary to nephrolithiasis. CT imaging suspected a crossed fused right-to-left renal ectopia, with the unusual finding of a single ureter crossing the midline and inserting into the bladder on the contralateral side. This anatomical variant should significantly influence clinical management and follow-up. We describe the diagnostic process, implications and embryological considerations of this rare condition. We highlight the importance of contrast-enhanced CT in evaluating suspected or confirmed urinary tract malformations.