The 22-year-old endured the distressing condition for five weeks before seeking help from doctors at Tallaght Hospital in Dublin.
Doctors who treated him first attempted compression with a special dressing for two weeks but this proved temporary and, as a soon as it was removed, the erection resumed.
He eventually had to have a blood vessel blocked in a non-invasive procedure although doctors could only find two previously documented reports of an unwanted prolonged erection after a straddle injury.
He made a full recovery and one month later there was no recurrence, with the patient reporting "satisfactory erection and intercourse".
The man, who was a competitive biker, had what is known as a "high-flow priapism" with a rigid erection, which happens without sexual stimulation, according to his doctors who reported on their unusual patient in the 'Irish Medical Journal'.
He had fallen astride his bicycle handlebar and suffered pain, bruising and swelling, all of which had cleared up within a few days.
However, the unwanted erection persisted; although when he was examined by doctors they found no signs of injury and apart from embarrassment he was not in discomfort.
The successful treatment involved inserting gel foam and four tiny platinum coils at an abnormal connection between an artery and a vein that supplied blood to the man's penis.
Consultant Intervention radiologist at Tallaght Hospital, Dr Ronan Browne described the treatment as "a great result for him. We were very happy with the outcome. It was an anxious time for the patient as it would be for any young man."
Although the procedure was minimally invasive, it involved "a high level of expertise".
Doctors had to insert a catheter, a flexible tube through a narrow opening, into the man's pelvic artery.
The delicate treatment saw the doctors inserting foam and coils into the artery, mindful that it carried a small risk of impotence.
However, other more invasive treatment had a higher risk of impotence. Dr Browne pointed out: "You have to get it right -- you don't want to block off too much blood supply."
Dave Jordan of mountainbiking.ie said: "The poor fella -- it sounds extremely unusual."