The corrosion inhibition and adsorption behavior of sodium alginate on AZ31 magnesium alloy in 3.5 wt% sodium chloride solution is investigated. The results show that sodium alginate can effectively inhibit the corrosion of AZ31 magnesium alloy. With increasing sodium alginate concentration, the inhibition efficiency initially increases and then decreases. It reaches a maximum of 90.00% at a sodium alginate concentration of 500 ppm. The inhibition is due to adsorption of sodium alginate on the bare magnesium alloy surface, where it mixes with magnesium hydroxide to form a more continuous and compact film, which retards the corrosion of the alloy.