Onychomatricoma (OM) is a rare benign fibroepithelial tumor originating from the nail matrix that often mimics onychomycosis both clinically and mycologically, leading to delayed diagnosis. We report the case of a 23-year-old male with a history of tuberous sclerosis who developed a solitary nail lesion on the second toe of the left foot. The lesion was initially misdiagnosed as onychomycosis and treated unsuccessfully. Dermoscopic findings suggested onychomatricoma, which was later confirmed by histopathological examination following total nail avulsion. The patient showed favorable evolution without recurrence. This represents the second reported case of an association between onychomatricoma and tuberous sclerosis. The case highlights the importance of comprehensive nail evaluation in patients with genodermatoses.