Co_0.5Ni_0.5Nb_xFe_2−xO₄ (0.00 ≤ x ≤ 0.10) nanoparticles (NPs) were prepared using the hydrothermal approach. The X-ray powder diffraction (XRD) pattern confirmed the formation of single-phase spinel ferrite. The crystallite size was found to range from 18 to 26 nm. The lattice parameters were found to increase with greater Niobium ion (Nb³⁺) concentration, caused by the variance in the ionic radii between the Nb³⁺ and Fe³⁺. Fourier transform infrared analysis also proved the existence of the spinal ferrite phase. The percent diffuse reflectance (%DR) analysis showed that the value of the band gap increased with growing Nb³⁺ content. Scanning electron microscopy and transmission electron microscopy revealed the cubic morphology. The magnetization analyses at both room (300 K, RT) and low (10 K) temperatures exhibited their ferromagnetic nature. The results showed that the Nb³⁺ substitution affected the magnetization data. We found that Saturation magnetization (M_s), Remanence (M_r), and the Magnetic moment ( $n_B$ ) decreased with increasing Nb³⁺. The squareness ratio (SQR) values at RT were found to be smaller than 0.5, which postulate a single domain nature with uniaxial anisotropy for all produced ferrites. However, different samples exhibited SQRs within 0.70 to 0.85 at 10 K, which suggests a magnetic multi-domain with cubic anisotropy at a low temperature. The obtained magnetic results were investigated in detail in relation to the structural and microstructural properties.