Abstract Finite element modeling was developed for buckling of an unbalanced fiberglass–rubber sandwich composite panels under hydrostatic pressure. The analysis was verified with tests on two sets of full-scale panels with different rubber cores. The model was composed of solid elements for the core, and shell elements with a neutral axis offset for the skins. An accurate description of the nonlinear rubber at strains <5% was crucial. The buckling pressures predicted by the model were 148 ± 7 and 323 ± 24 kPa for the two sets of panels, which agreed well with the measured values of 147 ± 2 and 317 ± 7 kPa, respectively. At low pressure, there was agreement of deflections and strains from the model to the measurement, but at high pressure the agreement was unsatisfactory. A secondary elastic instability was observed, occurring more suddenly in the model. The differences were ascribed to the lack of inclusion of defects in the model.