We demonstrate a 355 nm ultraviolet nanosecond laser produced by the type I phase-matched frequency doubling in a K3B6O10Br (KBOB) nonlinear optical crystal. The use of 5022 mu J, 710 nm pump pulses at 10 Hz repetition rate produces 4.273 ns, 355 nm, 717 mu J pulses that corresponds to a frequency doubling conversion efficiency of 14.3%. The energy instability of the frequency-doubled pulse train measured is less than +/- 1% for maximum output pulse energy (similar to 717 mu J). Our experimental results show that KBOB is a promising nonlinear optical crystal for generating ultraviolet nanosecond laser pulses through frequency doubling.