Potassium dihydrogen phosphate KHPO (KDP) single crystals, both pure and with incorporated titanium dioxide TiO nanoparticles, are ferroelectric materials with dipole structure used in nonlinear optics, optoelectronics, and acoustooptics. To date, a comprehensive analysis of structural features and defect states in the KDP and KDP/TiO matrix is absent in literature data. Pure KHPO (KDP) single crystals and KDP/TiO crystals with different TiO nanoparticles with anatase structure, synthesized by the sulfate and chloride methods, and with η-TiO structure, prepared by the sulfate method, grown by the temperature lowering method and cut from the pyramidal and prismatic growth sectors, have been investigated by the X-ray diffraction methods and by EDX spectroscopy. According to the energy-dispersive X-ray microanalysis, the sulfur content is higher in the sample with η-TiO and in the KDP/η-TiO crystal if compared with the samples with anatase and KDP/TiO (anatase). It was revealed that the sulfur content is higher in the prismatic growth sector if compared with the pyramidal one. Based on results of the X-ray single-crystal diffraction analysis, it is possible to assume the substitution of $$ \text{PO}_{4}^{3 - } $$ tetrahedra by $$ \text{SO}_{4}^{2 - } $$ ones in the KDP/TiO structures, which is greater in KDP/η-TiO. The general composition of the KDP/η-TiO crystal from the prismatic growth sector with the highest titanium content can be written as $$ (\text{K}_{0.950(1)} \square_{0.050} )\text{Ti}_{0.052\left( 2 \right)i} \left( {\text{H}_{2 - x}^{1 + } \square_{x} } \right)\left[ {\left( {\text{PO}_{4} } \right)_{y}^{3 - } \left( {\text{SO}_{4}^{2 - } } \right)_{1 - y} } \right] (\square \text{ - vacancies}). $$ It was first found that the Ti ions occupy the interstitial site (−0.048, 0.045, 0.550) in the KDP crystal matrix. Analysis of frequency-dependant dielectric characteristics revealed that the incorporation of TiO nanoparticles reduces the dielectric permittivity values of KDP/TiO samples compared with the pure KDP ones. It was found that the KDP/η-TiO sample has the smallest value of the dielectric permittivity. [ABSTRACT FROM AUTHOR]