A 355 nm ultraviolet laser with hydrothermal KTP is demonstrated. A high power ultraviolet laser with high beam quality at 355 nm is obtained by internally frequency tripling of a diode side-pumped acousto-optic (AO) Q-switched Nd:YAG laser. Type II phase-matched hydrothermal KTP and LBO crystals are used for the second harmonic generation and the third harmonic generation, severally. Using the V-shaped resonator cavity, under the pump power of 93.09 W, 4.133 W average output power at 355 nm is obtained at 4.9 kHz, corresponding to the pump-to-ultraviolet conversion efficiency of 4.44%.