We herein report the synthesis of a pure sphere-like spinel CoMn₂O₄ nanostructure using a facile and surfactant-free hydrothermal approach followed by a thermal decomposition of the as-prepared CoCO₃/MnCO₃ composite precursor. Various factors affecting the hydrothermal reaction of cobalt chloride, manganese chloride, and ammonium hydrogen carbonate have been investigated to synthesize a pure CoCO₃/MnCO₃ composite precursor. Calcination of the CoCO₃/MnCO₃ composite (synthesized using 0.4Co²⁺ : 0.6Mn²⁺ molar ratio) at 550 °C for 1 h gave the pure sphere-like spinel CoMn₂O₄ nanostructure product (∼16 nm), but the other carbonate composites (synthesized using other molar ratios) did not generate pure spinel CoMn₂O₄ on calcination. The as-prepared products were identified employing XRD, FT-IR, TEM, EDS, FE-SEM, zeta potential, TG, and BET analyses. The as-prepared spinel CoMn₂O₄ product showed high adsorption capacity (132 mg g⁻¹) for the removal of Reactive Black 5 (RB5) dye from aqueous media. The pseudo-second-order kinetics and Langmuir isotherm models described well the experimental adsorption results. The adsorption of RB5 dye on the as-prepared adsorbent is an endothermic, spontaneous, and physisorption process according to the calculated thermodynamic constants: ΔH⁰ (22.144 kJ mol⁻¹), ΔG⁰ (from −4.321 to −6.990 kJ mol⁻¹), and E_a (20.916 kJ mol⁻¹), respectively. The as-prepared spinel CoMn₂O₄ adsorbent showed high stability, reusability, and high adsorption capacity implying its efficiency in removing the RB5 textile dye from aqueous media.