In the present work, experimental shear tests using a custom-built picture-frame shear fixture have been conducted to investigate the in-plane pure-shear response of an HRP-C fiberglass/phenolic hexagonal honeycomb core under large deformation. Three-dimensional digital image correlation was used during the quasi-static tests to measure the homogenized surface strain fields in the core specimen. A new hyperelastic orthotropic material model was developed to capture the in-plane finite strain behavior of the honeycomb core. The picture-frame shear test on the core was simulated numerically using the homogenized material model, and predictions were compared with test data. Unlike the Hookean orthotropic material model, which exhibited softer responses in comparison to test data, good agreement was observed between the hyperelastic orthotropic model predictions and test results.