We present a community-led assessment of the capabilities of NASA's Wide Field InfraRed Survey Telescope (WFIRST) for solar system science. WFIRST will provide imaging and spectroscopic capabilities from 0.6-2.0 $\mu$m and will be a potential contemporary and eventual successor to JWST. Observations of asteroids, the giant planets and their satellites, Kuiper Belt Objects, and comets will be possible through both the Guest Investigator (GI) and Guest Observer (GO) programs. Surveys of irregular satellites and minor bodies, as well as time domain studies of variable surfaces and atmospheres, are well-suited for WFIRST with its 0.28 deg$^2$ field of view. Potential ground-breaking WFIRST investigations include detection of the first minor bodies orbiting in the Inner Oort Cloud, identification of additional Earth Trojan asteroids, and the discovery and characterization of asteroid binary systems similar to Ida/Dactyl. Previous use of astrophysics assets for solar system science and synergies between WFIRST, LSST, JWST, and the proposed NEOCam mission are discussed. We also provide a list of proposed minor modifications to the mission, including non-sidereal tracking of 30 mas/s and a K-band filter ($\sim$2.0-2.4 $\mu$m).
51 pages, 14 figures, 6 tables