<p>Pure and indium-substituted cobalt ferrite nanoparticles with composition CoFe₂₋ₓInₓO₄ (<i>x</i> = 0.00 and 0.5) were successfully synthesized via an environmentally friendly green tea-assisted route. The novelty of this work lies in the use of a sustainable bio-mediated synthesis approach combined with low-level In³⁺ substitution to tailor the structural, thermal, elastic, and magnetic characteristics of cobalt ferrite nanomaterials. X-ray diffraction confirmed the formation of a single-phase cubic spinel structure without detectable secondary phases, while microstructural analysis revealed lattice modification and grain refinement induced by indium incorporation. Morphological studies indicated reduced particle agglomeration and enhanced porosity in the doped sample. Thermal analysis demonstrated improved crystallization behavior and modified decomposition characteristics upon doping. FTIR results confirmed the characteristic metal–oxygen vibrational bands of the spinel structure and indicated changes in local bonding environments. Elastic parameter analysis suggested a slight reduction in lattice stiffness while maintaining structural stability. Magnetic measurements revealed enhanced saturation magnetization and remanence together with reduced coercivity and magnetic anisotropy, highlighting the beneficial role of In³⁺ substitution. The combined results demonstrate that indium doping provides an effective strategy for tuning the multifunctional properties of cobalt ferrite nanoparticles synthesized through a green and sustainable route, making them promising candidates for magnetic and advanced functional applications. Overall, the results demonstrate that indium substitution induces controlled lattice expansion, reduced crystallite size, and slight mechanical softening without disturbing the fundamental spinel framework, making these materials promising for applications requiring tunable structural and functional properties.</p>