The adsorption of an acid dye from water onto pristine and HCl-activated montmorillonites in fixed beds was investigated. Experiments were carried out as a function of liquid flow rate, initial dye concentration, and bed height. The adsorption capacity of acid dye onto pristine clay could be largely improved when the clay was activated by HCl. A mass transfer model that involves only two parameters, τ (50% breakthrough time) and k (adsorption rate constant), was proposed. This model could satisfactorily describe the measured breakthrough curves of acid dye in fixed beds (standard deviation <6%). It was shown that the value of τ decreased with increasing liquid flow rate. The effect of the type of clay (pristine, acid-activated) on the values of k, τ, and adsorption capacity was discussed, and the application potential of acid-activated clay for adsorption removal of acid dye from water was also demonstrated.