A diode-pumped picosecond 8-pass amplifier with a liquid-nitrogen-cooled Yb:YAG crystal has been developed. An average output power of 23.7 W with a near-diffraction-limited beam quality (M(2) < 1.2) was obtained at a pulse repetition rate of 80 kHz and a pulse duration of 11.7 ps. Average powers above 20 W were also obtained in the 30-80 kHz repetition rate range. The pulse energy reached almost 1 mJ at the 20 kHz repetition rate.