The dynamics of the ferroelectric phase transition in a SrBi2Ta2O9 film of 5.5 µm thickness deposited on sapphire was studied by means of time-domain terahertz transmission spectroscopy (30–300 K) and Fourier transform far-infrared transmission spectroscopy (300–950 K). The optical soft mode near 28 cm−1 (at 300 K) exhibits a weak monotonic softening down to 20 cm−1 at 950 K with no significant anomaly near either the ferroelectric or ferroelastic phase transition. An additional relaxation process below the phonon frequencies was resolved in the terahertz transmission spectra. The critical slowing down of its relaxation frequency could be responsible for the dielectric anomaly near the ferroelectric transition temperature. This indicates that the ferroelectric transition is predominantly of order–disorder type.