Removal of methylene blue (MB) from water using adsorbent developed from waste wool fiber has been investigated by batch adsorption experiments in the present work. Wool fiber waste (WF) was collected from a local Spinning and Weaving Company, cleaned and further treated with citric acid to increase its adsorption capacity toward basic dyes. Raw and treated WF were ball milled to produce wool powder (WP) and modified wool powder (MWP), respectively. The carboxyl content was increased steadily as the citric acid solution concentration increased. The introduction of the carboxyl groups also has been proved by streaming potential measurements. The effect of contact time, wool dose and dye solution pH on the adsorption process were studied too. The adsorption isotherm was examined using Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) models. The adsorption equilibrium was attained within 20 minutes with a maximum removal percentage of 70 and 78 for WP and MWP, respectively. The wool dose affects the adsorption process significantly, especially for MWP which removes a higher percentage of MB (86 %) compared to WP (75 %). Both Freundlich and D-R were well fitting the adsorption equilibrium data with R2 > 0.9. The values of Freundlich exponent factor (n) and D-R adsorption energy (E) reveal the chemisorption nature of the MB adsorption on both WP and MWP. This work demonstrated that, chemical and physical modifications of the waste wool fiber with citric acid has enhanced the adsorption of MB.