We present the results from a coincident seismic reflection/refraction grid conducted at the Lesser Antilles subduction zone near 16¿N. This paper focuses on the seismic refraction data and constraints these data place on the three-dimensional structure of the island arc backstop. We find that the backstop in this region contains considerable topography in both the strike and dip directions. Two ridges, each 25--35 km in length and ~10 km in width, rise 1--6 km above the adjacent basement. The eastern edge of one of the ridges deepens by ~4--6 km over a horizontal distance of 10 km and forms the eastern edge of the backstop. In contrast to the complex nature of the backstop, the adjacent accretionary wedge displays little lateral variability at large scales. This may be a consequence of the spatial scales involved: the backstop topography is ~10--35 km in width, while the accretionary wedge extends ~125 km from the deformation front to the backstop. The top of the subducting oceanic crust, as identified by an increase in velocities to 6--6.5 km/s, intersects the backstop at a depth of ~14--15 km. The updip limit of plate boundary seismicity is located 75--100 km west and downdip of the backstop. However, two earthquake clusters are observed at the intersection of the subducted Barracuda Ridge and Tiburon Ridge with the backstop, suggesting active deformation associated with the backstop edge at these locations.