Authors:Schmidt et alAbstract:We report the discovery and classification of SDSS J053341.43+001434.1 (SDSS0533), an early-L dwarf first discovered during a powerful ΔV less than −11 magnitude flare observed as part of the ASAS-SN survey. Optical and infrared spectroscopy indicate a spectral type of L0 with strong Hα emission and a blue NIR spectral slope. Combining the photometric distance, proper motion, and radial velocity of SDSS0533 yields three-dimensional velocities of (U,V,W)=(14±13,−35±14,−94±22) km s−1, indicating that it is most likely part of the thick disk population and probably old. The three detections of SDSS0533 obtained during the flare are consistent with a total V-band flare energy of ∼8×1033 ergs (Etot>6.2×1034 erg), placing it among the strongest stellar flares detected. The presence of this powerful flare on an old L0 dwarf may indicate that stellar-type magnetic activity persists down to the end of the main sequence and on older ultracool dwarfs.