The Arabia–India plate boundary—also called the Owen fracture zone—is perhaps the least-known boundary among large tectonic plates1, 2, 3, 4, 5, 6. Although it was identified early on as an example of a transform fault converting the divergent motion along the Carlsberg Ridge to convergent motion in the Himalayas7, its structure and rate of motion remains poorly constrained. Here we present the first direct evidence for active dextral strike-slip motion along this fault, based on seafloor multibeam mapping of the Arabia–India–Somalia triple junction in the northwest Indian Ocean. There is evidence for 12 km of apparent strike-slip motion along the mapped segment of the Owen fracture zone, which is terminated to the south by a 50-km-wide pull-apart basin bounded by active faults. By evaluating these new constraints within the context of geodetic models of global plate motions, we determine a robust angular velocity for the Arabian plate relative to the Indian plate that predicts 2–4 mm yr-1 dextral motion along the Owen fracture zone. This transform fault was probably initiated around 8 million years ago in response to a regional reorganization of plate velocities and directions8, 9, 10, 11, which induced a change in configuration of the triple junction. Infrequent earthquakes of magnitude 7 and greater may occur along the Arabia–India plate boundary, unless deformation is in the form of aseismic creep.