A betavoltaic cell, which directly converts beta particles into energy, is composed of a junction diode and a beta-emitting source. Because the cells can deliver electricity over a long operation life ranging from several years to a decade, they are promising devices for applications in remote locations such as outer space, deserts, and underground areas. Herein, we report efficient energy conversion using a diamond pn junction. We characterized the betavoltaic performance under electron-beam irradiation using scanning electron microscopy and observed an open-circuit voltage of 4.26 V, a fill factor of 0.85, and a semiconductor conversion efficiency of 28%. These are the best values reported thus far for betavoltaic cells. The efficiency is close to the theoretical Shockley–Queisser efficiency limit for betavoltaic cells.