Acute renal infarction is a rare condition that can progress to renovascular hypertension, chronic kidney disease, and end-stage kidney disease, depending on the severity. We present a case of a 30-year-old man with no significant past medical history who presented with sudden onset right flank pain. Initially pyelonephritis was suspected but it was later found to be acute renal infarction secondary to cardioemboli from atrial fibrillation.