We have obtained low-resolution optical spectra of three faint brown dwarf candidates in the Pleiades open cluster. The objects observed are Roque 12 (I_C=18.5), Roque 5 (I_C=19.7), and Roque 25 (I_C=21.2). The spectrum of Roque 25 does not show the strong TiO band heads that characterize the optical spectra of M-type stars, but molecular bands of CaH, CrH, and VO are clearly present. We classify Roque 25 as an early L-type brown dwarf. Using current theoretical evolutionary tracks we estimate that the transition from M type to L type in the Pleiades (age ~120 Myr) takes place at T_eff~2200 K or M~0.04 M_solar. Roque 25 is a benchmark brown dwarf in the Pleiades because it is the first known one that belongs to the L-type class. It provides evidence that the initial mass function (IMF) extends down to about 0.035 M_solar and serves as a guide for future deep searches for even less massive young brown dwarfs.