We use multispacecraft Magnetospheric Multiscale observations to investigate electric fields and ion reflection at a nonstationary collisionless perpendicular plasma shock. We identify subproton scale (5-10 electron inertial lengths) large-amplitude normal electric fields, balanced by the Hall term (J×B/ne), as a transient feature of the shock ramp related to nonstationarity (rippling). The associated electrostatic potential, comparable to the energy of the incident solar wind protons, decelerates incident ions and reflects a significant fraction of protons, resulting in more efficient shock-drift acceleration than a stationary planar shock.