Blue naevus is a rare lesion on genital mucosa and may cause confusion in differential diagnosis with other pigmented lesions. In this case report, a 39-year-old man presented with a sudden onset in adulthood of blue naevus on the glans penis. A biopsy confirmed the diagnosis. Due to the unusual presentation, the onset and the risk of turning invasive, a careful examination was performed in order to minimise any risk of misclassification with melanoma. Afterwards, the patient was followed in a dermatologic department every six months. To our knowledge, only few similar cases have been described in literature.