We present the first investigation of optomechanics in an integrated one-dimensional gallium phosphide (GaP) photonic crystal cavity. The devices are fabricated with a newly developed process flow for integration of GaP devices on silicon dioxide (SiO2) involving direct wafer bonding of an epitaxial GaP/AlxGa1-xP/GaP heterostructure onto an oxidized silicon wafer. Device designs are transferred into the top GaP layer by inductively-coupled-plasma reactive ion etching and made freestanding by removal of the underlying SiO2. Finite-element simulations of the photonic crystal cavities predict optical quality factors greater than 106 at a design wavelength of 1550 nm and optomechanical coupling rates as high as 900 kHz for the mechanical breathing mode localized in the center of the photonic crystal cavity. The first fabricated devices exhibit optical quality factors as high as 6.5 × 104, and the mechanical breathing mode is found to have a vacuum coupling rate of 200 kHz at a frequency of 2.59 GHz. These results, combined with low two-photon absorption at telecommunication wavelengths and piezoelectric behavior, make GaP a promising material for the development of future nanophotonic devices in which optical and mechanical modes as well as high-frequency electrical signals interact.