Crystalline beta barium borate (β‐BaB2O4) thin films have been successfully prepared in situ on several kinds of substrates by using opposed‐targets magnetron sputtering. The crystallinity of these films is highly dependent on the deposition temperature. Crystalline films start to form at 500 °C and gradually become c‐axis aligned as the temperature is increased. At 700 °C the film grown on sapphire substrates shows hundred‐micrometer‐sized domain structures with a single (006) x‐ray diffraction peak normal to the film surface. For a linearly polarized fundamental beam incident along the surface normal, the second harmonic intensity exhibits a sixfold azimuthal pattern with a d22 second harmonic coefficient of 1.2 pm/V.