The confinement of light in components with nanoscale cross-sections in nanophotonic circuits significantly enhances the magnitude of the optical forces experienced by these components(1,2). Here we demonstrate optical gradient forces between two nanophotonic waveguides, and show that the sign of the force can be tuned from attractive to repulsive by controlling the relative phase of the optical fields injected into the waveguides. The optical gradient force could have applications in optically tunable microphotonic devices and nanomechanical systems.