Stable passive Q-switching of a Tm: LiYF4 laser is obtained using polycrystalline Cr2+: ZnS as a saturable absorber. The achieved maximum pulse energy of 0.9 mJ and peak power of 65 kW for a pulse duration of ∼14 ns represent substantial improvement and highest values for a passively Q-switched diode-pumped Tm laser operating at ∼1.9 μm.