The wide-field monitor (WFM) is one of the three instruments on the “Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays” (STROBE-X) mission, which was proposed in response to the National Aeronautics and Space Administration’s 2023 call for a probe-class mission. The WFM is a coded-mask camera system that would be the most scientifically capable wide-angle monitor ever flown. The WFM will anchor X-ray time domain astronomy, at the all-sky level, for the 2030s. The field of view covers one-third of the sky, to 50% mask coding, and the energy sensitivity is 2 to 50 keV. The WFM will identify new X-ray transients for rapid observations with the two pointed instruments of STROBE-X. In addition, the WFM will capture spectral/timing changes in known sources with data of unprecedented quality. WFM data will uniquely advance scientific knowledge for diverse classes in high-energy astrophysics, including X-ray bursts that coincide with gravitational wave detections, gamma-ray bursts (GRBs) and their transition from prompt emission to afterglow, subluminous GRBs that may signal shock breakout in supernovae, state transitions in accreting compact objects and their jets, bright flares in fast X-ray transients, accretion onset in transitional pulsars, and coronal flares from many types of active stars.