We report on the absorption, resonance Raman, and Raman excitation profile spectra of mass-selected terbium dimers in argon matrixes. An absorption band found between 620 and 680 nm was assigned to terbium dimers. The Raman excitation profile of dimer frequencies resembles closely this absorption spectrum. Unlike other lanthanide dimers, resonance Raman spectra of terbium dimer, obtained by excitation into this region with tunable dye laser radiation, gives two progressions with one up to 7 Stokes transitions, and the other up to 4 transitions. We interpret these to represent two distinct electronic states X and A. For the ground (X) state, we obtain omega (e) = 137.6 +/- 0.4 cm(-1) with omega (e)x(e) = 0.31 +/- 0.05 cm(-1), leading to a spectroscopic dissociation energy of 1.9 +/- 0.3 eV, force constant k(e) = 0.88 +/- 0.01 mdyn/Angstrom. The first electronic (A) state has an origin at 313.9 cm(-1) with a progression in omega (e) = 136.3 +/- 0.2 cm(-1) and omega (e)x(e) = 0.14 +/- 0.05 cm(-1), leading to an excited-state spectroscopic dissociation energy of 4.1 +/- 1.2 eV.