The temperature- and frequency-dependent dielectric susceptibility of $\sf\small KD_2AsO_4$ was studied in the ferroelectric phase. A dispersion which is related to the dipole reversal was found in the high-frequency range ($10^7$ Hz). The dispersion is described by a Gaussian distribution of relaxation times which show Vogel-Fulcher temperature dependences. The high value of the dielectric constant below the phase transition is related to the mobility of the domain walls. This mobility steeply decreases near the freezing temperature $T_f$ and therefore a sudden decrease in the susceptibility takes place. A non-linear superposition of the piezoelectric resonance and the relaxational dispersion was observed.