State-of-the-art advances in nanoscale optomechanics allow light to be guided in free-standing waveguides or resonators. In closely spaced devices, the coupling between the guided lightwaves gives rise to an optical force known as the 'optical bonding force'. Indeed, attractive optical force has been observed in substrate coupled devices. According to recent theoretical predictions, the optical force should show bipolar behaviour depending on the relative phase between in-plane coupled lightwaves. So far, such an in-plane optical force has not been measured. Here, we experimentally demonstrate a bipolar optical force between planarly coupled nanophotonic waveguides. Both attractive and repulsive optical forces are obtained. The sign of the force can be switched reversibly by tuning the relative phase of the interacting lightwaves. This highly engineerable force of bipolar nature could be used as the operation principle for a new class of planar light force devices and circuits on a CMOS-compatible platform.