We demonstrate single-frequency stimulated Brillouin lasing in short fiber Fabry-Perot resonators. A sub-milliwatt threshold for lasing is observed in resonators with high quality factors of ∼1.5 × 108 and effective resonator lengths as short as ∼5 mm. The strong dispersion of fiber Bragg gratings results in unequal frequency separations for neighboring pairs of longitudinal modes with separation differences larger than the Brillouin gain bandwidth, thereby avoiding cascaded lasing. Our achievement will enable compact, robust, efficient narrow-linewidth light sources.