Posterior urethral valves (PUVs) in patients with Down syndrome (DS) have been previously described. However, no case of PUVs has been reported in an adult patient with DS. We report on a 40-year-old man with DS, who presented with two severe urinary infections and obstructive symptoms. He was found to have type 3 PUVs associated with bladder neck sclerosis. PUV resection and bladder neck incision were done. Because PUVs seem to be more frequent in patients with DS, complete investigations should be performed, even in adults with DS, in the case of urinary infection or lower urinary tract symptoms.