A tunable narrow linewidth UV laser near 308 nm is necessary for highly sensitive hydroxyl (OH) radical measurement. We demonstrated a high-power fiber-based single frequency tunable pulsed UV laser at 308 nm. The UV output is generated from the sum frequency of a 515 nm fiber laser and a 768 nm fiber laser, which are harmonic generations from our proprietary high-peak-power silicate glass Yb- and Er-doped fiber amplifiers. A 3.50 W single frequency UV laser with 100.8 kHz pulse repetition rate, 3.6 ns pulse width, 34.7 µJ pulse energy, and 9.6 kW peak power has been achieved, which represents the first demonstration, to the best of our knowledge, of a high-power fiber-based 308 nm UV laser. With temperature control of the single frequency distributed feedback seed laser, the UV output is tunable for up to 792 GHz at 308 nm.