We report the discovery of BOSS-EUVLG1 at z = 2.469, by far the most luminous, almost un-obscured star-forming galaxy known at any redshift. First classified as a QSO within the Baryon Oscillation Spectroscopic Survey, follow-up observations with the Gran Telescopio Canarias reveal that its large luminosity, MUV ≃ -24.40 and log(LLyα/erg s-1) ≃ 44.0, is due to an intense burst of star formation, and not to an active galactic nucleus or gravitational lensing. BOSS-EUVLG1 is a compact (reff ≃ 1.2 kpc), young (4-5 Myr) starburst with a stellar mass log(M*/M☉) = 10.0 ± 0.1 and a prodigious star formation rate of ≃1000 M☉ yr-1. However, it is metal- and dust-poor [12 + log(O/H) = 8.13 ± 0.19, E(B - V) ≃ 0.07, log(LIR/LUV) < -1.2], indicating that we are witnessing the very early phase of an intense starburst that has had no time to enrich the ISM. BOSS-EUVLG1 might represent a short-lived (<100 Myr), yet important phase of star-forming galaxies at high redshift that has been missed in previous surveys. Within a galaxy evolutionary scheme, BOSS-EUVLG1 could likely represent the very initial phases in the evolution of massive quiescent galaxies, even before the dusty star-forming phase.