A boron-doped polycrystalline diamond film was deposited on a molybdenum substrate and resistively heated in a high vacuum environment. The thermally excited emission current as a function of cathode temperature was measured. This phenomenon, known as thermionic emission, is characterized by electron emission arising from a surface at elevated temperatures and is described by the Richardson equation. The observed thermionic emission current from the diamond sample followed the Richardson equation from which a work function of 4.43 eV and a Richardson constant of ∼60 A/cm2K2 were observed. This indicates boron-doped polycrystalline diamond behaves as an interesting thermionic emitter for possible energy conversion and other electron emission applications.