We present the first detection of an X-ray flare from an ultracool dwarf of spectral class L. The event was identified in the EXTraS database of XMM-Newton variable sources, and its optical counterpart, J0331-27, was found through a cross-match with the Dark Energy Survey Year 3 release. Next to an earlier four-photon detection of Kelu-1, J0331-27 is only the second L dwarf detected in X-rays, and much more distant than other ultracool dwarfs with X-ray detections (photometric distance of 240 pc). From an optical spectrum with the VIMOS instrument at the VLT, we determine the spectral type of J0331-27 to be L1. The X-ray flare has an energy of EX,F 3c 2
7 1033 erg, placing it in the regime of superflares. No quiescent emission is detected, and from 2.5 Ms of XMM-Newton data we derive an upper limit of LX, qui < 1027 erg s-1. The flare peak luminosity (LX, peak = 6.3
7 1029 erg s-1), flare duration (\u3c4decay 48 2400 s), and plasma temperature ( 4816 MK) are similar to values observed in X-ray flares of M dwarfs. This shows that strong magnetic reconnection events and the ensuing plasma heating are still present even in objects with photospheres as cool as 3c2100 K. However, the absence of any other flares above the detection threshold of EX, F 3c 2.5
7 1032 erg in a total of 3c2.5 Ms of X-ray data yields a flare energy number distribution inconsistent with the canonical power law dN/dE 3c E-2, suggesting that magnetic energy release in J0331-27 - and possibly in all L dwarfs - takes place predominantly in the form of giant flares
