Pronounced strain-rate-dependent Poisson's ratio (mu) and stress (sigma) are observed for a fully swollen gel (60 mm in length and 1.2 mm in thickness) in solvents when greatly stretched in tensile experiments with a wide range of crosshead speed (nu), across S orders of magnitude from an extremely slow nu of 0.17 mu m/s. The strain-rate dependence is the result of the stretching-driven swelling during deformation and not viscoelastic effects. This feature is unique to polymer gels that behave as semiopen systems, i.e., systems that can exchange solvent: with its surroundings. When v is so low that the time scale of stretching ((epsilon) over dot(0)(-1) where (epsilon) over dot(0) = nu/l(0) is the initial strain rate and l(0) the initial length) is comparable to the characteristic time of swelling (tau), mu and sigma become strongly dependent on nu:mu becomes less than 1/2 and sigma becomes smaller than the value corresponding to the sufficiently high nu (satisfying the relation (epsilon) over dot(0-1), <> tau), the strain-induced swelling is equilibrated at all times during stretching: mu, (approximate to 0.25) and sigma are at equilibrium values, independent of nu. A model to describe the nu dependence of mu and stress-strain relation is developed from the existing model for swelling dynamics under a small step strain on the basis of the Boltzmann superposition principle. The model well captures the main features of the experimental results. The results of the present study also provide fundamental information about the effects that the actuation rate has on the mechanical performance of gel-based soft actuators because this effect becomes pronounced at easily attainable deformation speeds for microfabricated gels and fiber gels with relatively short characteristic time for swelling.