The Himalayan plate boundary, because it is entirely subaerial, is both the most dramatic and the most accessible to direct observation of all active convergent boundaries on Earth. The shape of this boundary can be described as a small circle of radius 1696 ± 55 km, centered at long 91.6° ± 1.6°E and lat 42.4° ± 2.1°N for the extent of the arc between long 77.2° and 92.1°E. The pole of this small circle is consistent whether seismicity, topography, or stress state is used to define the position of the tectonic boundary. The defined small circle also coincides with a peak in microseismicity, the maximum horizontal strain rate, and a peak in the vertical velocity field. This quantitative definition of a stable, curved tectonic boundary is a prerequisite to modeling the dynamics of curvature in convergent arcs and applying appropriate boundary conditions to other regional models.