We report a rare case of a primary malignant lymphoma of the prostate, which presented as urinary retention in a 32-year-old male patient. A digital rectal examination revealed an enlarged hardly prostate. B-cell lymphoma arising in the prostate was diagnosed by a prostate biopsy and confirmed by an immunohistochemical examination. The patient received doxorubicin-based combination chemotherapy, which showed a good outcome in terms of both decreasing the size of the tumor and improving the associated symptoms during 9 months of follow-up. To our knowledge, there are only a few reported cases of a primary malignant lymphoma arising in the prostate. We recommend that the digital rectal examination be used in young male patients who reveal abnormal urinary voiding function.