Polyethylenimine-based modified coal fly ash (CFA-PEI) is synthesized by the wet impregnation method. The material was found to be an excellent adsorbent for removal of cationic (Malachite Green; MG) and anionic (Reactive Red 2; RR2) dyes from aqueous solution. CFA-PEI was characterized by various sophisticated instruments such as FTIR spectroscopy, C-13 MAS NMR spectroscopy, XRD, elemental analysis, Brunauer-Emmett-Teller (BET) isotherm, TGA, FESEM, and HRTEM. The FTIR spectra reveal that the N-containing functional group is introduced on the surface of CFA-PEI, which plays a vital role in the removal of these dyes. The adsorption behavior of the CFA-PEI toward toxic dyes is evaluated by investigating the effect of solution pH, contact time, solution temperature, adsorbent dose, and concentration of dye solution. The obtained adsorption isotherm and the kinetics data of both MG and RR2 are in compliance with the Langmuir adsorption isotherm and pseudo-second-order kinetics, respectively. Moreover, excellent adsorption capacity on CFA-PEI of dyes (q(m): 174.83 mg/g for MG and 316.75 mg/g for RR2) is attributed to various factors (H-bonding, electrostatic interaction). CFA-PEI also has high regeneration capacity toward these dyes from aqueous solution. Finally, the advantage of the adsorbent lies in its low cost.