The optical switching of X-rays using laser-induced crystal lattice expansion is described. Irradiation of a gallium arsenide (GaAs) crystal using picosecond laser pulses shifts the Bragg angle through the lattice expansion with a response time of a few hundred picoseconds. A single pulse was extracted from the synchrotron radiation pulse train using a double-crystal arrangement of GaAs, in which the two crystals were irradiated by way of two successive laser pulses with an appropriate time delay.