Optical UBVRI photometry and medium-resolution spectroscopy of the Type Ib supernova SN 2009jf, during the period from ∼ −15 to +250 d, with respect to the B maximum are reported. The light curves are broad, with an extremely slow decline. The early post-maximum decline rate in the V band is similar to SN 2008D; however, the late-phase decline rate is slower than other Type Ib supernovae studied. With an absolute magnitude of MV=−17.96 ± 0.19 at peak, SN 2009jf is a normally bright supernova. The peak bolometric luminosity and the energy deposition rate via the 56Ni →56Co chain indicate that ∼0.17+0.03−0.03 M⊙ of 56Ni was ejected during the explosion. The He i 5876 Å line is clearly identified in the first spectrum of day ∼ −15, at a velocity of ∼16 000 km s−1. The [O i] 6300–6364 Å line seen in the nebular spectrum has a multipeaked and asymmetric emission profile, with the blue peak being stronger. The estimated flux in this line implies that ≳1.34 M⊙ oxygen was ejected. The slow evolution of the light curves of SN 2009jf indicates the presence of a massive ejecta. The high expansion velocity in the early phase and broader emission lines during the nebular phase suggest it to be an explosion with a large kinetic energy. A simple qualitative estimate leads to the ejecta mass of Mej= 4–9 M⊙ and kinetic energy EK= 3–8 × 1051 erg. The ejected mass estimate is indicative of an initial main-sequence mass of ≳20–25 M⊙.