The National Polar-orbiting Operational Environmental Satellite System (NPOESS) ushers in a new era of multispectral satellite radiometry to the operational platform. Combining several key strengths of contemporary operational and research-grade visible/infrared observing systems (drawing from the Advanced Very High Resolution Radiometer (AVHRR), the Operational Linescan System (OLS), and the Moderate-resolution Imaging Spectroradiometer (MODIS)), the NPOESS Visible/Infrared Imager/Radiometer Suite (VIIRS) represents a paradigm shift to current operational capabilities. In particular, the VIIRS Day/Night Band (DNB; drawing legacy from the OLS) holds great promise for unprecedented capabilities on the nighttime side the NPOESS orbit, particularly when used in a multispectral context. Unlike the OLS, the DNB will be fully calibrated with high radiometric resolution afforded by three stages of gain. In this way, applications based on reflected moonlight analogous those derived from daytime visible-band measurements of reflected sunlight should become possible—provided that the top-of-atmosphere down-welling lunar flux (a highly variant quantity over the course of the lunar cycle and season) is known. This paper describes the development of a lunar spectral flux database, rendered at 1 nm spectral resolution, for use in quantitative VIIRS/DNB applications. Provided as a function of sun/earth/moon geometry, the database can be convolved with the satellite sensor response function to provide the system-input information required to relate upwelling radiance to reflectance. The database, which has been validated against numerous point data provided by the astronomy community and vicarious satellite sensor calibration exercises, will be made available to the research community to assist in leveraging this unique NPOESS capability for advancing nighttime environmental applications.