The spontaneous emission spectrum and the small‐signal gain have been measured in a low‐energy, waveguide‐mode free‐electron laser. Measurements are carried out at microwave frequencies (25–40 GHz) in anX‐band rectangular waveguide with a low‐energy (450–600 keV, &ggr;∼2), low‐current (∼1.5 A), low‐energy spread (&Dgr;E/E≪10−4) relativistic photoelectron beam. The measured spontaneous emission spectrum agrees well with the homogeneously broadened spectrum of the TE10waveguide mode. The measured small‐signal gain spectrum is in good agreement with the theoretical prediction calculated under the assumption of operation in the low‐gain Compton regime.