A selenite chloride, Na₂Cu₇(SeO₃)₄O₂Cl₄ (1), was prepared via a conventional hydrothermal method. Na₂Cu₇(SeO₃)₄O₂Cl₄ crystallizes in the triclinic space group P and features an isolated reverse triangular dipyramid Cu₇, which is assembled with two corner-shared Cu₄ tetrahedral units. Magnetic measurements suggest that 1 shows the spin-frustration effect with antiferromagnetic ordering at ∼5 K, while an unusual magnetization plateau is observed at an applied field of >4 T.