Extensive optical and near-infrared observations of the Type Ib supernova (SNIb) 1999dn are presented, covering the first year after explosion. These new data turn this object, already considered a prototypical SNIb, into one of the best observed objects of its class. The light curve of SN 1999dn is mostly similar in shape to that of other SNeIb but with a moderately faint peak (MV =−17.2 mag). From the bolometric light curve and ejecta expansion velocities, we estimate that about 0.11 M⊙ of 56Ni were produced during the explosion and that the total ejecta mass was 4-6 M⊙ with a kinetic energy of at least 5 × 1051 erg. The spectra of SN 1999dn at various epochs are similar to those of other stripped envelope SNe showing clear presence of H at early epochs. The high explosion energy and ejected mass, along with the small flux ratio Ca ii/O i measured in the nebular spectrum, together with the lack of signatures of dust formation and the moderate metallicity environment is not inconsistent with a single massive progenitor (M ZAMS≥ 23-25 M⊙) for SN 1999dn.