Diamond has been developed as a material for the detectionof charged particles by ionization. Its radiation hardness makes itan attractive material for detectors operated in a harsh radiationenvironment e.g. close to a particle beam as is the case for beammonitoring and for pixel vertex detectors. Poly-crystalline chemicalvapor deposition (CVD) diamond has been studied as strip and pixeldetectors so far. We report on a first-time characterization of asingle-crystal diamond pixel detector in a 100 GeV particle beam atCERN. The detectors are made from irregularly shaped single crystalsensors, 395 μm thick, mated by bump bonding to a front-endreadout IC as used in the ATLAS pixel detector with pixel sizes of50 × 400 μm2. The diamond sensors show excellentcharge collection properties: full collection over the entiredetector volume, clean and narrow signal charge distributions with aS/N value of ∼ 100 and a hit detection efficiency of (99.9±0.1)%. The measured spatial resolution for particlesunder normal incidence in the shorter pixel direction is (8.9±0.1) μm.