A 6-year-old intact male Doberman pinscher presented with a thin soft stool and urinary signs. On radiography, three masses were observed in the caudal abdomen. One mass was described as a large, round, mineralized mass, with an "eggshell" appearance. The second mass was located caudal to the mineralized mass, and the third mass was located between the two masses. The second and the third masses had a soft tissue density. Ultrasonography was performed to identify the features and the origins of the masses. The first mass contained a large amount of anechoic fluid and had a thin wall; it was determined to be a cyst. Strong acoustic shadowing artifact was created by the mineralized cystic wall. Caudal to the cyst, the second mass was shown to be the prostate. A hypoechoic stalk connected the prostate and the cyst, which represented the cyst as a paraprostatic cyst with mineralization. The third mass was the normal urinary bladder. The cyst was removed surgically and confirmed by histopathologic examination. This report described typical clinical findings, diagnostic imaging, and treatment of a paraprostatic cyst.