Virtually all global energy forecasts include an expectation that bioenergy will be a substantial future energy source. However, the scale of this potential resource remains poorly understood because of uncertain land availability and yield expectations. Here, we used climate-constrained, satellite-derived net primary productivity data computed for 110 million square kilometers of terrestrial plant production as an upper-envelope constraint on primary bioenergy potential (PBP). We estimated the maximum PBP to realistically range from 12% to 35% of 2009 global primary energy consumption, with yield potential ranging from 6.6 to 18.8 megajoules per square meter per year—roughly four times lower than previous evaluations. Our results highlight many recent bioenergy evaluations as overoptimistic, which we attribute to a failure to adequately apply biophysical constraints in estimates of yield potential. We do not advocate bioenergy production at the levels reported in this analysis; instead, we simply report the ceiling for PBP based on current planetary productivity.