A novel polymorph of RbCuCl₃ (rubidium copper trichloride), denoted ɛ-RbCuCl₃, has been prepared by chemical vapour transport (CVT) from a mixture of CuO, CuCl₂, SeO₂ and RbCl. The new polymorph crystallizes in the orthorhombic space group C222₁. The crystal structure is based on an octahedral framework of the 4H perovskite type. The Rb⁺ and Cl⁻ ions form a four-layer closest-packing array with an ABCB sequence. The Cu²⁺ cations reside in octahedral cavities with a typical [4 + 2]-Jahn–Teller-distorted coordination, forming four short and two long Cu—Cl bonds. ɛ-RbCuCl₃ is the most structurally complex and most dense among all currently known RbCuCl₃ polymorphs, which allows us to suggest that it is a high-pressure phase, which is unstable under ambient conditions.