—Due to the wide range of uses of water-soluble polymers in commercial products, water contamination by polymers has become a serious environmental concern. Adsorption onto an acidic clay, obtained from Tsunagi mine, Niigata, Japan, of water-soluble polymers from aqueous solutions was investigated as a means of purifying water. Poly(vinylpyrrolidone) (PVP) was used as a sample polymer in an attempt to find optimal conditions for extracting the greatest proportion of polymer from the aqueous solution. The adsorption isotherms at lower equilibrium concentrations were of type L, indicating a strong affinity between the acidic clay and PVP. A larger amount of PVP was adsorbed when a higher-molecular-weight PVP (comparison between MWs of 160,000 and 10,000) was used. From the Langmuir equation, the adsorption capacity was calculated as 0.029 g/g clay for the adsorption of poly(vinylpyrrolidone) from an aqueous solution. The adsorption of PVP from a NaCl solution (simulated sea water) was also possible though the amount of adsorption was slightly less than from an aqueous solution. A polymer removal efficiency of >90% was achieved when 200 mg of the acidic clay was added to 50 mL of 0.001 wt.% PVP aqueous solution. The acidic clay was also used for adsorption of poly(ethylene glycol), poly(vinyl alcohol), and polyacrylamide from aqueous solutions.