β-catenin plays an important role in hair morphogenesis. Previously, the nuclear and cytoplasmic localizations of β-catenin were identified in hair-matrix cells. To evaluate β-catenin expression in the nail matrix, we obtained human nail units. Immunohistochemistry for β-catenin was used to evaluate sections of normal nail units and of sections from a single case of onychomatricoma. In the nail unit, β-catenin was expressed in the nucleus and cytoplasm of the suprabasal nail-matrix cells. Of the other epithelial-cell types, only the cell membrane was β-catenin-positive. In the nail tissue from the onychomatricoma case, β-catenin was expressed in the nucleus and cytoplasm of the upper epithelial layers. Our result suggests that β-catenin plays an important role in nail formation. In addition, β-catenin expression in onychomatricoma supports the presence of nail-matrix cells in this condition. To our knowledge, this is the first report of β-catenin expression in the nucleus and cytoplasm of the nail matrix.