Potassium dihydrogen phosphate KH2PO4 (KDP) single crystals, both pure and with incorporated titanium dioxide TiO2 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/TiO2 matrix is absent in literature data. Pure KH2PO4 (KDP) single crystals and KDP/TiO2 crystals with different TiO2 nanoparticles with anatase structure, synthesized by the sulfate and chloride methods, and with eta-TiO2 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 eta-TiO2 and in the KDP/eta-TiO2 crystal if compared with the samples with anatase and KDP/TiO2 (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 tetrahedra by ones in the KDP/TiO2 structures, which is greater in KDP/eta-TiO2. The general composition of the KDP/eta-TiO2 crystal from the prismatic growth sector with the highest titanium content can be written as It was first found that the Ti4+ 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 TiO2 nanoparticles reduces the dielectric permittivity values of KDP/TiO2 samples compared with the pure KDP ones. It was found that the KDP/eta-TiO2 sample has the smallest value of the dielectric permittivity.