An infected paraprostatic cyst originating from the prostate parenchyma was diagnosed in a seven-yearoldmale American Staffordshire terrier with dysuria. The diagnosis was based on the history, the clinicaland ultrasonographic findings, bacteriological culture, extensive histopathological examination and aDNA-test. Treatment consisted of the surgical removal of a great part of the cyst and the omentalizationof the remnant, castration and antibiotic therapy. Immunohistochemistry demonstrated an urothelial liningof the cyst. A DNA-test excluded persistent Müllerian duct syndrome (PMDS). The cyst had an openconnection with the urethra and probably originated from an embryonic remnant located in the prostaticgland. The dog had a prosperous recovery but the dysuria and urinary incontinence persisted. At fourmonths postoperatively, a new paraprostatic cyst was diagnosed by ultrasound. Aspiration of its contentrevealed infected urine.