<p>This study investigates how substituting Cr³⁺ with Zr⁴⁺ affects the structural, morphological, and optical properties of CoCr_2−xZr_xO₄ nanoparticles, with their spectroscopic characteristics examined through electron spin resonance (ESR) spectroscopy. As Zr⁴⁺ substitution increased, the crystallite size decreased significantly (25.43 to 12.11&#xa0;nm), while the lattice parameter showed a slight increase (8.307 to 8.319&#xa0;Å). SEM and TEM confirmed a quasi-spherical, agglomerated morphology, and EDX verified successful zirconium incorporation without impurities. Optical characterization using UV–Vis and PL spectroscopy revealed a quantum confinement effect, with band gap widening (3.02–3.61&#xa0;eV) as crystallite size decreased. The linear refractive index and its dispersion were analyzed using the Wemple–DiDomenico single-oscillator model, and nonlinear optical properties, specifically third-order susceptibility <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\({\chi }^{(3)}\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mrow> <mi>χ</mi> </mrow> <mrow> <mo stretchy="false">(</mo> <mn>3</mn> <mo stretchy="false">)</mo> </mrow> </msup> </math></EquationSource> </InlineEquation> and nonlinear refractive index <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\({n}_{2}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>n</mi> <mn>2</mn> </msub> </math></EquationSource> </InlineEquation>, were evaluated as functions of Zr content. As the zirconium (Zr) content increases, the nanoparticles exhibited a marked reduction in their nonlinear optical response, dropping from 3.61 × 10^–11 to 0.097 × 10^–11 esu. This decline was accompanied by a decrease in crystallite size and a corresponding increase in microstrain. Electron spin resonance (ESR) measurements were performed to examine the effect of Zr⁴⁺ substitution on key spectroscopic parameters, including the <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(g\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>g</mi> </math></EquationSource> </InlineEquation> -factor, (<InlineEquation ID="IEq4"> <EquationSource Format="TEX">\({H}_{r}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>H</mi> <mi>r</mi> </msub> </math></EquationSource> </InlineEquation>), peak-to-peak linewidth (<InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(\Delta {H}_{pp}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="normal">Δ</mi> <msub> <mi>H</mi> <mrow> <mi mathvariant="italic">pp</mi> </mrow> </msub> </mrow> </math></EquationSource> </InlineEquation>), and spin relaxation times. Overall, CoCr_2−xZr_xO₄ nanoparticles exhibited a tunable combination of linear optical, nonlinear optical, and spin relaxation properties, allowing their behavior to be tailored for applications in optoelectronic devices, magnetic resonance imaging, and spin-based memory.</p>