An ultrasmall single-electron transistor has been made by scaling the size of a fin field-effect transistor structure down to an ultimate limiting form, resulting in the reliable formation of a sub-5 nm Coulomb island. The charge stability data feature the first exhibition of three and a half clear Coulomb diamonds at 300 K, each showing a high peak-to-valley current ratio. Its charging energy is estimated to be more than one order magnitude larger than the thermal energy at room-temperature. The hybrid literal gate integrated by this single-electron transistor combined with a field-effect transistor displays >5 bit multiswitching behavior at 300 K with a large voltage swing of ∼1 V.