Dense nanocrystalline BaTiO3 ceramics are prepared in a single step by the Cold Sintering Process at 300 °C, under a uniaxial pressure of 520 MPa for 12 h using a molten hydroxide flux. Transmission electron microscopy reveals a dense microstructure with sharp grain boundaries. The average grain sizes are 75−150 nm depending on the flux amount. The dielectric permittivity is 700–1800 at room temperature at 106 Hz, with a dielectric loss, tan δ ∼ 0.04. The difference in permittivity and phase transition behavior are explained in terms of the intrinsic size effect of the BaTiO3. The nanocrystalline BaTiO3 ceramics still shows a macroscopic ferroelectric switching via a hysteresis loop. This work demonstrates that cold-sintering process could enable the densification of ferroelectric oxides in a single step. Futhermore, comparable dielectric properties to reported values for nanocrystalline grains are obtained, but at this time, with the lowest processing temperatures ever used.