Newly synthesized cerium oxide was successfully obtained by the hard templating route. The optimal As(III) and As(V) adsorption onto the studied adsorbent was reached for the initial pH of 4.0 and a contact time of 10 h. The highest static adsorption capacities for As(III) and As(V) were 92 mg g−1 and 66 mg g−1, respectively. The pseudo-second-order model was well fitted to the As(III) and As(V) experimental kinetics data. The Langmuir model described the As(III) and As(V) adsorption isotherms on synthesized material. The adsorption mechanism of the studied ions onto the synthesized cerium oxide was complex and should be further investigated. The optimal solid–liquid ratio during the proposed aqueous extraction of inorganic As from the Fucus vesiculosus algae was 1:50. The optimal dosage of the synthesized cerium oxide (0.06 g L−1) was successfully applied for the first time for inorganic As removal from the aqueous algal extract.