Australia and India are conventionally thought to be contained in a single plate divided from an Arabian plate by the Owen Fracture Zone. We propose instead that motion along the nearly aseismic Owen Fracture Zone is negligible and that Arabia and India are contained within a single Indo-Arabian plate, divided from the Australian plate by a diffuse boundary. This boundary, which trends E--W from the Central Indian Ridge near Chagos Bank to the Ninetyeast Ridge, and north along the Ninetyeast Ridge to the Sumatra Trench, is a zone of concentrated seismicity and deformation heretofore characterized as ''intraplate''. Plate motion inversions and an F-ratio test show that relative motion data along the Carlsberg Ridge are fit significantly better by the new model. The misclosure of the Indian Ocean triple junction is reduced by 40%. The rotation vector of Australia relative to Indo-arabia is consistent with the seismologically observed ~2 cm/yr of left-lateral strike-slip along the Ninetyeast Ridge, N--S compression in the Central Indian Ocean, and the N--S extension near Chagos, This boundary, possibly initiated in late Miocene time, may be related to the opening of the Gulf of Aden and the uplift of the Himalayas. The convergent segment of this boundary may represent an early stage of convergence preceding the initiation of subduction.