The effect of swelling on the true stress-strain response of a hydrogel was evaluated using a uniaxial tensile test, and the response was represented by the constitutive equation based on the molecular chain network theory. The experimental results clarified that the initial elastic modulus was reduced by an increase in the degree of swelling. Two different constitutive equations, where the swelling stretch was introduced in different forms, were proposed to predict the effect of swelling. The obtained results of two equations underestimated and overestimated the effect of the swelling on the true stress-strain curve. This result indicates that the mechanical response of the swollen hydrogel can accurately be predicted using the parameter related to the swelling stretch.