Computational models that evaluate high-rate loading scenarios to the thorax rely on material properties of the impacted soft tissues. The shear behaviour of these soft tissues, including viscoelastic stress relaxation, needs to be accounted for in an accurate model. Pure shear tests at high rate and high shear strain were performed on porcine dorsal skin, ventral skin, liver and lung tissue post-mortem. Synthetic gelatin was subjected to the same shear tests, to evaluate its validity as a tissue surrogate. Instantaneous elastic shear properties of the tissues were determined, and their stress relaxation over short (1 ms) and long (20 s) timescales. Dorsal skin tissue was found to have the highest shear stiffness, followed by ventral skin, liver and lung. Synthetic 20% gelatin approximates the instantaneous elastic shear properties of porcine dorsal skin but does not show the same viscoelastic relaxation behaviour. Synthetic 10% gelatin behaved similarly to 20% gelatin in stress relaxation, but with significantly reduced shear stiffness. Shear moduli of biological tissues increase with increased shear strain, suggesting a non-linear model is appropriate for computational purposes. A future determination of shear properties for human tissues will allow for a quantitative evaluation of using porcine tissue as a surrogate.