We report stable orthogonally polarized domains in high-density polariton solitons propagating in a semiconductor microcavity wire. This effect arises from spin-dependent polariton-polariton interactions and pump-induced imbalance of polariton spin populations. The interactions result in an effective magnetic field acting on polariton spin across the soliton profile, leading to the formation of polarization domains. Our experimental findings are in excellent agreement with theoretical modeling taking into account these effects.