Luciferases are widely used as sensitive reporters in various fields ranging from basic biology to medical diagnosis, public health, and food inspection. Scientists have isolated novel luciferases from bioluminescent organisms and concentrated on improving their brightness and thermostability. Recently, small bright luciferases such as artificial luciferase (ALuc) (21 kDa), NanoLuc (19 kDa), GLuc (18 kDa), and TurboLuc (16 kDa) have been reported. However, smaller, brighter, and more stable luciferases are desired for further applications. Here, we constructed the smallest and bright mutant of ALuc, named "picALuc" (13 kDa). picALuc retained the luminescence activity of the full-length ALuc; moreover, its brightness and thermostability were at the same levels as NanoLuc. Furthermore, we showed the advantage of picALuc for the bioluminescence resonance energy transfer-based assay due to its smallness. Our development has opened the door for wider and more practical applications of luciferases.