This work presents the design and test results of a charged-particle solid-state detector with ultrafast signal response based on polycrystalline chemical-vapor-deposition (pCVD) diamond as active detector material and a high-bandwidth RF amplifier. We tested the detector at the Indiana University Cyclotron Facility Bloomington, IN, in a proton beam with a kinetic energy ranging from 55 to 200 MeV. The detector signals showed an average pulsewidth of 1.38 ns, which enables single-particle counting at instantaneous rates approaching the gigahertz range. The detector operated with a signal-to-noise ratio of 7 : 1 for 200-MeV protons and a single-particle detection efficiency up to 99%.