We study the dynamic elastic behavior of potassium thiocyanate (KSCN) in the temperature region of the order–disorder improper ferroelastic phase transition using dynamic mechanical analysis (0.05–40 Hz) and resonance ultrasound spectroscopy (100–600 kHz). The low frequency data show—in addition to the intrinsic phase transition anomalies—softening in a- and b-directions below Tc, which results from movements of ferroelastic domain walls under dynamic stress. In contrast to many other ferroelastic materials (LaAlO3, PbZrO3, SrTiO3, etc.), the domain wall motion in KSCN freezes already at a temperature below Tc–20 K. The corresponding increase of the domain wall relaxation time τDW with decreasing temperature can be well fitted by a Vogel–Fulcher law τDW=τ0exp[Ea/kB(T−TVF)] with τ0≈10−7 s, Ea≈0.035 eV, and TVF≈368 K, indicating domain glass behavior. The high frequency elastic moduli (∼f2) (100–600 kHz) do not show any precursor softening with decreasing temperature due to the large order parameter relaxation time τη. In contrast to the high frequency elastic moduli, the corresponding losses Q−1 exhibit peaks at Tc=415 K, which may be explained by order parameter fluctuations.