A radio frequency (RF) grain mass and constituent measurement system utilized onboard a combine harvester includes an RF sensor subsystem for capturing RF sensor readings of a harvested grain within an area of the combine harvester. A memory stores an RF characteristic database, which contains RF characteristic testing data collected for tested grain samples over one or more tested frequency ranges. A controller, operably coupled to the RF sensor subsystem and to the memory, is configured to: (i) receive the RF sensor readings from the RF sensor subsystem; (ii) determine grain mass and a first constituent content of the currently-harvested grain based, at least in part, on an analytical comparison between the RF sensor readings and the RF characteristic testing data; and (iii) perform at least one action in response to determining the grain mass and the first constituent content of the harvested grain.