Using the method of time-resolved X-ray diffractometry in a three-crystal scheme, the lattice deformation of a rubidium biphthalate (С8H5RbO4) crystal was measured in an external electric field. Under the action of an external pulsed electric field along the [001] polar direction, the piezoelectric moduli d31, d32, and d33 were determined independently using three reflections 400, 070, and 004; the obtaind values are –32.8 ± 0.6, 12.8 ± 0.3, and 21.8 ± 1.2 pC/N, respectively. A good agreement was found between the values of the piezoelectric moduli obtained in this work and the values obtained earlier by the quasi-static method.