Grafting of poly(para-, ortho- and meta-phenylenediamine (PPDA)) onto electrospun carbon nanofibers was performed via oxidative polymerization of o-, p-, m-PDA onto the surface of ECNFs to produce PPDA-g-ECNFs. The well characterized PPDA-g-ECNFs and their precursors were applied as effective nanoadsorbent for the removal of Coomassie brilliant blue R-250 (CBB) from aqueous solutions. The obtained results indicated that the surface modification of ECNFs by conjugated polymers enhanced its adsorption capacity with respect to CBB and was dependent on the nature of surface functionalization, concentration of CBB, pH, and contact time. The equilibrium isotherm and kinetic parameters on the adsorption of CBB onto the functionalized ECNFs were evaluated and analyzed by non-linear isotherm and kinetic models. The results of the kinetic and isotherm studies revealed that the Elovich kinetic model and the Redlich–Peterson isotherm model were more suitable to describe the adsorption of CBB from an aqueous solution. Furthermore, the thermodynamic parameters exhibited spontaneity of CBB adsorption and its endothermic characteristic, thereby demonstrating the high efficiency of the introduced adsorbents for CBB adsorption. A possible mechanism of adsorption was suggested wherein electrostatic interaction, π–π interactions, and intermolecular H-bonding control the CBB adsorption. Display omitted •Poly(phenylenediamine) grafted electrospun carbon nanofibers (PPDA-g-ECNFs) were successfully prepared and characterized.•PPDA-g-ECNFs were used as an effective nanoadsorbent for the removal of toxic dye.•The effect of different parameters on the adsorption was investigated in detail.•The adsorption capacity of PPDA-g-ECNFs is higher than most reported results.•The mechanism was systematically discussed and proposed with kinetics, isotherms and thermodynamics.