BACKGROUND: Priapism is a very rare complication of malignancy and is usually accompanied by locally advanced or widely metastatic disease. We describe a case of priapism arising in a 46-year-old male with localised rectal cancer that was responding to therapy. CASE PRESENTATION: This patient had just completed two weeks of neoadjuvant, long-course chemoradiation when he presented with persistent painful penile erection. Assessment and diagnosis were delayed for more than 60 h, and although a cause could not be determined from imaging, a near complete radiological response of the primary rectal cancer was seen. His symptoms were refractory to urologic intervention and were associated with extreme psychological distress. He re-presented shortly thereafter with extensively metastatic disease in the lungs, liver, pelvis, scrotum, and penis; additionally, multiple venous thromboses were identified, including in the dorsal penile veins. His priapism was not reversible and was associated with a considerable symptom burden for the remainder of his life. His malignancy did not respond to first-line palliative chemotherapy or radiation, and his clinical course was further complicated by obstructive nephropathy, ileus, and genital skin breakdown with a suspected infection. We initiated comfort measures, and he ultimately died in hospital less than five months after his initial presentation. CONCLUSION: Priapism in cancer is usually related to tumour infiltration of the penis and corporal bodies resulting in poor venous and lymphatic drainage. The management is palliative and can include chemotherapy, radiation, surgical shunting, and potentially penectomy; however, conservative penis-sparing therapy may be reasonable in patients with limited life expectancy.