A kind of saturable absorber (SA) based on layered MoSe2 nanosheets has been fabricated successfully by an ultrasound-assisted liquid phase exfoliation method. The as-prepared MoSe2 SA was characterized and employed as Q-switch in a diode-pumped Tm-doped crystalline all-solid-state laser at around 2 μm. A stable passively Q-switched 2 μm laser with a maximum pulse repetition rate of 158 kHz and a minimum pulse width of 520 ns was successfully realized. A maximum average output power of 1.19 W, corresponding to a maximum pulse energy of 12.7 μJ and pulse peak power of 21.8 W, has been obtained. It was the first demonstration of MoSe2 SA in realizing a solid-state 2 μm pulsed laser, to the best of our knowledge. The results confirmed the excellent ability and promising potential of MoSe2 SA as an optical modulator in generating high repetition rate and short pulsed laser at the midinfrared spectral band.