Conjunctival leiomyosarcoma is a very rare soft tissue malignancy. Herein, we describe a conjunctival leiomyosarcoma case in a patient with another rare disease, xeroderma pigmentosum. The 27-year-old single-eyed xeroderma pigmentosum patient complained of exophytic mass covering the ocular surface in her left eye. A vascular, hemorrhagic mass covering the entire ocular surface of the left eye was identified on the examination. Thus, total mass excision surgery was performed. The pathological diagnosis was compatible with conjunctival leiomyosarcoma. Additional chemotherapy, radiotherapy, or surgery were not accepted by the patient. No recurrence or metastasis was observed during the 5-year follow-up. Both primary conjunctival leiomyosarcoma and xeroderma pigmentosum are very rare diseases. Conjunctival masses in xeroderma pigmentosum patients should be approached carefully, and histopathological examination is warranted. For conjunctival leiomyosarcoma, early diagnosis, localized, unspread disease, and complete resection provide the best prognosis.