Metal-induced allergic contact dermatitis, particularly nickel, affects over 10% of the general population. Herein, chitosan–glycerol gel as protective barrier formulation was synthesized by neutralization reaction with an aim to reduce metal-ion diffusion into the skin to prevent allergy. Active functional groups in chitosan–glycerol gel were able to capture allergenic metal ions present in artificial sweat solution. The efficacy of the barrier formulation against nickel-ion penetration was evaluated ex vivo using pig skin. We found that the percutaneous absorption of nickel ion reduced by ∼98% when chitosan–glycerol gel was used as a barrier formulation.