Glass fiber-reinforced (GFRP) cell-core sandwich structures are increasingly used in bridge deck and roof construction. GFRP cell-core sandwiches are composed of the outer GFRP face sheets, a foam core and a grid of GFRP webs integrated into the core in order to reinforce the shear load capacity. One of the critical failure modes is shear wrinkling, a local buckling failure in the sandwich webs due to shear loading. Shear wrinkling is a biaxial compression-tension wrinkling problem and, for this reason, the numerous results of pure compressive wrinkling research are not necessarily applicable. The details and results of in-plane biaxial compression-tension wrinkling experiments on GFRP sandwich laminates, stabilized by a polyurethane foam core, are presented. It is shown that an increasing transverse tension load significantly decreases the wrinkling load. These results are confirmed by finite element calculations.