The Ocean Color Instrument (OCI) is the primary payload on NASA’s Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission. Its primary purpose is to enable new scientific studies of ocean biology, aerosols, and clouds. This paper describes the design of the instrument and its radiometric performance as measured during the prelaunch characterization campaign. OCI will be the first radiometer to provide hyperspectral (340nm895nm) daily global coverage of top-of-atmosphere radiances. Seven multispectral bands cover wavelengths from 940nm to 2260nm. The spatial resolution is about 1.2km. OCI performance is optimized for ocean color applications, with a focus on high signal-to-noise ratio (SNR) at low radiance levels and high radiometric accuracy.