We demonstrate the third-order nonlinear optical properties of Cr4+:Y3Ga5O12 ( Cr4+:YGG) and Q-switched lasers with Cr4+:YGG as the saturable absorber for the first time to our knowledge. The third-order nonlinear properties, including the optical Kerr nonlinearity and saturable absorption, were systematically measured and analyzed in detail by using a Z-scan technique. The measured data show that Cr4+:YGG has a large nonlinear refractive index, ground-state absorption cross section, and excited-state absorption cross section in contrast to Cr4+:Y3Al5O12 ( Cr4+:YAG). With a Nd:YGG crystal as the gain medium and a Cr4+:YGG crystal as the saturable absorber, the passively Q-switched laser was performed. The shortest pulse width and largest pulse energy were achieved at the absorbed pump power of 8 W with the values of 9.1 ns and 26.1 mu J, respectively, corresponding to the average output power of 0.87 W and peak power of 2.9 kW. The results indicate that Cr4+:YGG is an available and promising optical switcher for pulsed lasers.