According to one embodiment, a thin-film transistor and a method of manufacturing the same achieve size reduction of the thin-film transistor while using an oxide semiconductor layer. The oxide semiconductor layer includes a channel region, a source region, and a drain region. A gate electrode is arranged at a position spaced from the channel region of the oxide semiconductor layer so as to face the channel region. A source electrode is electrically connected to the source region of the oxide semiconductor layer. A drain electrode is electrically connected to the drain region of the oxide semiconductor layer. An undercoat layer adjoins the source region and the drain region of the oxide semiconductor layer. A hydrogen blocking layer has a hydrogen concentration lower than that in the undercoat layer and separates the undercoat layer and the channel region of the oxide semiconductor layer.