The prediction of rib fractures using a computational thorax model remains an important and elusive challenge. The finite element models currently available do not properly include the contribution of the intercostal muscles (ICM). The mechanical behavior of these muscles, which are composed of several layers with their own fiber orientations, is not sufficiently known yet. However, documenting the orientation of the muscle fibers in each layer is not trivial. Therefore, the goal of this study was to evaluate whether the addition of the fiber orientations in a constitutive model of the ICM would improve the prediction of the ICM mechanical response. To do so, tensile tests were performed on three ICM samples harvested from one cadaver. After preconditioning, the samples were tested to fracture under quasi‐static loading. The load and applied displacement were measured, and an optical system (ARAMIS) was used to measure the strain field on the external layer. Material parameters and the fiber orientations were estimated through an optimization process and an inverse FE approach, where the experimental stress–strain profiles were fitted to those obtained from the FE simulations performed specifically for the samples. The prediction of the ICM mechanical response improved when proper fiber orientation was taken into account.