Onychopapilloma is an uncommon benign neoplasm of the nail matrix and nail bed. The clinical feature of onychopapilloma is not specific, and the majority of cases present clinically as localized longitudinal erythronychia, exceptionally as leukonychia or melanonychia. It is characterized histologically by distal subungal hyperkeratosis and nail matrix metaplasia of the nail bed with marked papillomatosis. A 50-year-old man presented with 3 year history of a longitudinal pigmented streak of the left index fingernail. He reported tenderness of the lesion. He had been treated with antifungal agent for 3 months, but showed no response after 1 year of follow up. He had a history of ankylosing spondylitis. Total excision was done, and the histologic findings showed papillomatous nail matrix squamous epithelial hyperplasia and thick keratin layer in the distal nail bed. A periodic acid-Schiff test did not reveal fungal elements. The patient showed good response without recurrence. We concluded that these findings were consistent with onychopapilloma presenting as longitudinal melanonychia, which was not a common clinical feature. Our case adds onychopapilloma to the list of conditions that can induce melanonychia.