We have measured near normal incidence far-infrared (FIR) reflectivity spectra of a single crystal of TbMnO₃ from 10 K to 300 K in the spectral range of 50 cm^-1 –700 cm^-1. Fifteen transverse optic (TO) and longitudinal optic (LO) modes are identified in the imaginary part of the dielectric function $\varepsilon_{2} (\omega)$ and energy loss function Im($−1/\varepsilon(\omega))$, respectively. Some of the observed phonon modes show anomalous softening below the magnetic transition temperature $T_{N}$ ($\sim 46$ K). We attribute this anomalous softening to the spin-phonon coupling caused by phonon modulation of the superexchange integral between the Mn³⁺ spins. The effective charge of oxygen ($Z_{O}$) calculated using the measured LO–TO splitting increases below $T_{N}$.