Nature Materials, Published online: 11 November 2026; doi:10.1038/s41563-026-02802-4In spintronics, the spin of particles is used to transfer information but the propagation distances are short, whereas photons carry spin further but lack the nonlinear interactions necessary for logic gates. Now, polariton waveguides built from transition metal dichalcogenides enable spin propagation of up to 100 μm at room temperature and maintain large polariton nonlinearities.