Before the 2-Micron All-Sky Survey (2MASS) began, only six objects were known with spectral types later than M9.5 V. In the first 371 deg² of actual 2MASS survey data, we have identified another 20 such objects spectroscopically confirmed using the Low Resolution Imaging Spectrograph (LRIS) at the W. M. Keck Observatory. Because the TiO and VO bands, which dominate the far-optical portions of late-M spectra, disappear in these cooler dwarfs, we define a new spectral class "L" in which metallic oxides are replaced by metallic hydrides and neutral alkali metals as the major spectroscopic signatures. We establish classification indices and type all 25 L dwarfs. The 26th "post-M9.5" object—Gl 229B—is the prototype of a methane-dominated spectral class, which we propose as class "T." At least five of the 20 2MASS L dwarfs show the 6708 Å lithium doublet at low resolution, the strongest having an equivalent width of 18.5 Å. For objects this cool, the presence of lithium proves that they are substellar. Two other 2MASS objects appear to have lithium lines at the limit of our detectability, which if verified means that at least one-third of our L dwarfs are bona fide brown dwarfs. All of the 2MASS brown dwarfs discovered so far have J-K_s > 1.30. We have not yet, despite deliberately searching for them, found any brown dwarfs with colors resembling Gl 229B (J-K_s ≈ -0.1).