An important challenge in current microelectronics research is the development of techniques for making smaller, higher-performance electronic components. In this context, the fabrication and integration of ultrathin high-κ dielectrics with good insulating properties is an important issue. Here, we report on a rational approach to produce high-performance nanodielectrics using one-nanometer-thick oxide nanosheets as a building block. In titano niobate nanosheets (TiNbO5, Ti2NbO7, Ti5NbO14), the octahedral distortion inherent to site-engineering by Nb incorporation results in a giant molecular polarizability, and their multilayer nanofilms exhibit a high dielectric constant (160–320), the largest value seen so far in high-κ nanofilms with thickness down to 10 nm. Furthermore, these superior high-κ properties are fairly temperature-independent with low leakage-current density (<10−7 A cm−2). This work may provide a new recipe for designing nanodielectrics desirable for practical high-κ devices.