Uterus masculinus (persistent Mullerian duct) is a vestigial embryological remnant of the paramesonephric duct system in males and has been associated with clinical signs such as dysuria, incontinence, tenesmus and urethral obstruction in dogs. The radiological appearance of cystic uterus masculinus in dogs has been described previously with the aid of retrograde positive or negative contrast cystography. The purpose of this retrospective study was to describe ultrasonographic features of confirmed or presumed uterus masculinus in a group of dogs with confirmed or presumed disease. Ultrasonographic findings were recorded based on a consensus opinion of two readers. A uterus masculinus was defined as cylindrical when no lumen was observed and tubular when it had lumen that was filled with anechoic fluid. Six dogs met the inclusion criterion with a mean age of 8 years and 9 months. Uterus masculinus appeared as single (four dogs) or two (two dogs) horn-like, tubular (four dogs) or cylindrical (two dogs) structures, originating from the craniodorsal aspect of the prostate gland and extending cranially. The walls of the uterus masculinus were isoechoic to the urinary bladder wall. The diameter of the observed uterus masculinus varied from 0.3 cm to 1 cm. The length of the uterus masculinus varied from 2 cm to 6.5 cm but the cranial terminal end was not identified in two dogs. Concomitant prostatomegaly was seen in five dogs (83.3%) and urinary tract infection was noted in three dogs (50%). Findings indicated that uterus masculinus should be included as a differential diagnosis for male dogs with these ultrasonographic characteristics.