The causes of purple patches on the penis include glans inflammation, condyloma acuminatum, genital herpes, syphilis and so on. 1. Glans inflammation: due to various factors triggering inflammation of the glans, at this time there can be skin damage, so there are purple patches appear. 2. Condyloma acuminatum: when the penis is infected by human papilloma virus, these viruses can damage the penis leading to the proliferation of penile tissues, so purple patches appear. 3. Genital herpes: If the penis is infected by herpes viruses, these viruses can irritate the penis and cause swelling of the penile tissues, resulting in purple patches. 4. Syphilis: If the penis is infected by syphilis spirochetes, these spirochetes can damage the penile tissue and cause a syphilis rash, which appears as purple patches. Purple patches on the penis may also be caused by other factors, it is recommended that patients go to the regular hospital in time to seek treatment, and strive for early recovery.