<p>Spintronics, whereby electron spin is harnessed for carrying and processing information, could play an important role in the future of information technology. However, despite ongoing research efforts, establishing a materials platform that suits spin-optronics, particularly one that operates effectively at ambient temperatures, continues to represent a challenge. Recent advancements in transition metal dichalcogenides are opening up new opportunities, with exciton-polaritons in these materials being promising for the development of spintronic customizable devices that function at ambient temperatures. Although transition metal dichalcogenide polaritons have shown promising potential, spin-anisotropic nonlinearities have been missing. Here we demonstrate the absence of spin-anisotropic interaction in a monolayer WS₂ microcavity at room temperature and show how spin anisotropy can be recovered by engineering double WS₂ layer structures with varied interlayer spacing. We attribute this phenomenon to a distinctive feature in exciton–polariton physics: layer-dependent polariton–phonon coupling. We use theoretical calculations of the phonon electrostatic potentials finding a drastically different coupling strength for single and double monolayer samples and discuss qualitatively how this explains the observed spin-anisotropic response. This is further consistent with experiments on multi-WS₂ layer samples and the identification of a critical separation distance, above which an effective single monolayer spin-anisotropic response is recovered, both in experiment and theory. Our work lays the groundwork for the development of spin-optronic polaritonic devices at room temperature.</p>