Biomass plastics are attracting research attention owing to the growing environmental problems caused by plastic products. In this study, we investigated the effects of counter cations on the gas-permeation properties of hydrogen, oxygen, nitrogen, carbon dioxide, and methane in the membrane state of alginic acid (Alg) and sodium alginate (Na-Alg), in which the hydrogen atom of the carboxy group of Alg from seaweed is replaced with sodium, as well as ammonium alginate (NH4-Alg), in which it is replaced with ammonium ions. These polysaccharide membranes exhibited permeation behavior based on a solution-diffusion mechanism similar to that of common polymeric membranes. Using Alg as the standard, the counter cations in the Na-Alg and NH4-Alg membranes affected the aggregation structure of the polymer chains and contributed to the construction of a densely packed membrane structure. These polysaccharide membranes can be used as gas-barrier layers for earth-friendly industrial applications such as packaging.