Abstract Co 0.25 Ni 0.25 Zn 0.25 Cu 0.25 Ag x Fe 2-x O 4 (0.00 ≤ x ≤ 0.10) nanospinel ferrites (Ag → CoNiZnCu (x ≤ 0.10) NSFs) have been synthesized via a one-pot sol–gel method. XRD analysis was applied to prove the phase formation for each product. The morphologies were confirmed via SEM/TEM. This study introduced a detailed analysis of the electrical and dielectric properties of ion-substituted spinel ferrites. AC/DC conductivity and complex impedance spectroscopy were both used to understand how substitution of Ag + affects the charge-transport properties of these (NSFs). The unsubstituted NSF (x = 0.00) had very high resistivity (GΩ range) and was found to be mainly affected by one process of relaxation that occurred due to high resistive grain boundaries. After substitution with Ag, the DC conductivity increased dramatically by several orders of magnitude, which correlated with a massive reduction in resistance of the grain boundaries. The overall electrostatic and dielectric properties of these NSFs changed drastically with Ag + ion substitution as well; this change was due to the presence of the effect known as colossal permittivity (ϵ′ > 104), which is explained by the Maxwell–Wagner theory of interfacial polarization. All types of analysis (using both the complementary impedance and electric modulus formalisms, including Nyquist plots) proved that by separating the different electrical responses from the grains and grain boundaries of the substituted NSFs, these materials are heterophase-natured. Therefore, it can be concluded that Ag + ion substitution is a very effective way to tune the properties of grain boundaries, resulting in a measurable difference in the total electrical/dielectric response of the NSFs.