[en] This paper reports a suppressed ionic conductivity in a KTiOPO₄ (KTP) crystal grown from a potassium-rich K₄P₂O₇ flux. Potassium-rich KTP is shown to have an ionic conductivity, which is reduced by one order of magnitude relative to normal flux-grown KTP and is explained by an improved potassium-oxygen stoichiometry. However, it is found that the frequency dispersion of the conductivity is almost identical in form to that of conventional KTP, showing that the fundamental Debye-like origin of the conductivity is unchanged as the number of potassium vacancies is decreased. This behaviour is contrasted with that of Rb-doped KTP, in which the conductivity is reduced by a factor of more than 100 and the dispersion is drastically changed to resemble a disordered system rather than a Debye-like relaxor. Potassium-rich KTP is also found to have a reduced low-frequency dielectric constant, a contracted Cole-Cole plot and a lowered ferroelectric coercive electric field of only 3.0(±0.1) kV mm^-1 at room temperature. Successful periodic poling of a potassium-rich KTP crystal to produce a grating of 60 μm period is demonstrated at room temperature for the first time. X-ray topographs proving the existence of periodic domains throughout the bulk of potassium-rich KTP in both the usual [100] grating orientation and the unconventional [010] orientation are presented