A 51-year-old Caucasian man consulted for a visual loss in the left eye due to corneal extension of a conjunctival melanoma. This conjunctival melanoma arose from primary acquired melanosis with atypia at the temporal corneoscleral limbus. The patient was treated using a combination of surgical excision with physical treatment by ocular proton therapy. Progression remained under control 11 months after treatment: no local tumour recurrence or metastasis was observed. Primary acquired melanosis with atypia must be regarded as a premalignant melanocytic lesion. Based on this case report, the authors focus on primary acquired melanosis and its risk of transformation to a conjunctival malignant melanoma.