The adsorption capability of eosin Y as a model anionic dye on natural halloysite nanotubes (HNTs) and sulfuric acid-treated HNTs as a function of acid treatment time (1 h, 3 h, and 5 h) was examined. Scanning electron microscopy revealed that natural HNTs had a very uniform surface, whereas acid-treated HNTs had a rough surface with structural defects, which increased with acid treatment time. The total specific pore volume and total surface area of the acid-treated HNTs increased due to formation of nanopores in the HNTs via dissolution of the inner AlO6 octahedral layer. With acid treatment, the surface ξ-potentials were positively shifted from -42.9 mV (for the natural HNTs) to -1.9, -3.0, and +1.2 mV after 1, 3, and 5 h, respectively. The adsorption amount (qe) of eosin Y on natural HNTs and the three acid-treated HNTs was 2.3, 125.5, 118.9, and 118.9 mg g-1, respectively, implying that the adsorption capacity of acid-treated HNTs is ~50 times higher than that of natural HNTs. In this study, we clearly demonstrated that acid-treated HNTs can be used as highly efficient nanomaterials for removal of dyes from wastewater containing anionic dyes.