Potassium nitrate has a rectangular hysteresis loop and is thought to be a promising material for nonvolatile ferroelectric memory. However, its polar phase is observed in a narrow temperature range. This paper deals with an effect of ammonium nitrate NH4NO3 on the dielectric properties of potassium nitrate. Thermal dependencies of the linear dielectric permittivity ε and the third-harmonic coefficient g3 for potassium nitrate and polycrystalline binary (KNO3)1–x(NH4NO3)x system (x = 0.025, 0.035, 0.050, 0.100) in the temperature range between 300 and 460 K have been investigated. A temperature range extension of the ferroelectric phase with increasing the x value was revealed. The ferroelectric phase was retained in the (KNO3)1–x(NH4NO3)x composites (x = 0.050, 0.100) down to room temperature. A thermodynamic model for the ammonium-nitrate effect on the temperature of reconstructive phase transition II → I in potassium nitrate was suggested.