In this letter, we report the discovery of the first young early L-dwarf in the Serpens cloud. It is obscured by more than ten magnitudes of visual absorption and was found during a near infrared (NIR) photometric survey of a 50 square arcmin area in the Serpens cloud, deep enough to probe the substellar domain. After selection from NIR colour-magnitude and colour-colour diagrams, its substellar nature has been confirmed by NIR spectroscopy at the VLT. We estimate the mass of this brown dwarf to be ~ 0.05 Msun and its age to be ~ 3.5 Myr. From its NIR indices, we estimate its present spectral type to be L0-L3, and using a model its future spectral type to be T. This is the first young brown dwarf ever found deeply embedded in a star formation region. Based on observations collected with NTT/SOFI and VLT/ISAAC instruments at the European Southern Observatories, La Silla and Paranal, Chile (ESO Programmes P63.L-0227 and P65.L-0637).