We demonstrate high-efficiency ultraviolet (UV) laser generation in a nonlinear optical crystal BaAlBO3F2 (BABF); 355 nm UV light was generated by using a type I BABF crystal as a sum-frequency mixing of the fundamental light and the second harmonic from a picosecond Nd:YAG laser operated at a repetition rate of 10 Hz with a pulse duration of 20 ps. An output of 0.635 mJ for the third harmonic was obtained. The conversion efficiency was 26.4%. As far as we know, it was the first time that third-harmonic generation in BABF was achieved. Obviously, the BABF crystal is a promising UV nonlinear optical material because of its relatively large nonlinear coefficient and nonhygroscopicity.