The removal of arsenic from groundwater and mine drainage is a globally important issue. Montmorillonite (Mont), a layered clay mineral, is expected to function as an adsorbent for arsenic dissolved as oxoacid ions because its edge sites are positively charged. However, Mont alone has a negatively charged face, which leads to self-aggregation and makes it difficult to stably provide the edge faces as adsorption sites. To enhance the adsorption of arsenate ions, Mont face–PEI–Mont face aggregates (PMPs) were prepared by suspending Mont in water with polyethyleneimine (PEI) as a cationic binder. When Mont and PEI were mixed at a 10:1 (mg) ratio, aggregation of Mont was observed, and the amount of As(V) adsorbed increased threefold compared to Mont alone. However, when an excess amount of PEI was added (Mont:PEI = 1:1, mg), the surface charge increased from -40 mV for Mont alone to +1.6 mV. Microscopic observation revealed that the Mont layer structure had collapsed, and the adsorption capacity for As(V) was lost. This indicates that PEI itself does not contribute to As(V) adsorption.