Structural, morphological, magnetic, and dielectric investigations are carried out in Nb-doped (viz., for x equal to 0.0-0.4 by wt%) nanocrystalline Ni-Zn ferrites synthesized by hydrothermal method. XRD, IR studies infer the growth of nano structures and Fe2O3 phase. Nb ions segregate at grain boundaries. Crystallite size (D) varies from 62 to 18 nm with x. FTIR absorption exhibits split at higher x. Saturation magnetization witnesses an overall decrease with x. Trends of M (s)(x) are explained by nature of dopant, preferential replacement, surface canting (Y-K angles) and lattice contraction. Variations of coercive field (H (c)) and D infer single-domain to multi-domain morphological transformation. Occurrence of superparamagnetism (SPM) is predicted for zeroed values of H (c)and for a critical concentration x (crit) equal to 0.321. Enhanced Nb5+-Fe2+ binding with dopant (x up to 0.2) results for decrease in dielectric constant , loss factor tan delta and increase in resistivity rho. Lowered ac conductivity is attributed to blockade of path by Nb5+ ions in the vicinity of B-sites. Relatively higher rho (similar to 10(8) Omega cm) and lower loss (tan delta similar to 10(-2)-10(-3)) evinced for 10 kHz. Enhanced core loss is realized with x manifested as lowered H (c) and tan delta to usher their utility in high-frequency applications.