Arsenic contamination in natural water has become a global issue because of arsenic's high toxicity, accumulation in the human body, and carcinogenicity. In this study, a new yttrium-manganese binary composite was developed by a one-step coprecipitation method. The mean diameter of the adsorbent was 6.3 μm, and the point of zero charge was 7.1. The adsorbent had a chemical formula of Y5Mn6O6(OH)12(CO3)5·5H2O according to the results obtained from the analysis of the element and functional group from X-ray photoelectron spectroscopy (XPS) study. The field emission scanning electron microscopy study showed that the adsorbent had a loose structure and was composed of nanosized flakes. The adsorption process was pH-dependent. The removal efficiency of arsenate by the adsorbent was much higher than that of arsenite. The optimal adsorption efficiency of arsenate was obtained at pH 6.0. The kinetics study showed that adsorption equilibrium of arsenate was reached within 25 h. The fit of the experimental data of the adsorption isotherm by the Langmuir model was better than that of the Freundlich model. The maximum arsenate adsorption capacity of 279.9 mg As/g was achieved at pH 7.0, much higher than most reported adsorbents. The presence of fluoride, sulfate, bicarbonate, phosphate, and humic acid had less influence on the arsenate adsorption, while the adsorption capacity was still above 220 mg As/g under the maximum concentration of coexisting substances. Fourier transform infrared spectroscopy and XPS analysis indicated that the hydroxyl group on the adsorbent played a more important role in the arsenate uptake.