Strong light–matter coupling is a powerful mechanism to engineer materials properties and a platform to study polariton physics. Excitons in organic crystals are interesting candidates for the investigation of light–matter coupling due to the large magnitude and well-defined orientation of their transition dipole moments. We demonstrate the coupling of excitons in tetracene crystals to optical modes in open cavities formed by anisotropic arrays of plasmonic nanoparticles and investigate the coupling strength as a function of the alignment of the exciton dipole moment to the cavity field. The anisotropy of the cavity and the crystal provides a practical method to tune the light–matter coupling strength from weak to the onset of the strong coupling regime, by rotating the crystal with respect to the plasmonic array. The possibility to control the coupling within a single excitonic material, paves the way to study the effects of the coupling strength on polariton physics, such as exciton-polariton dynamics, transport, or condensation.