Two previously unrecognized concomitant polymorphic structures of the copper complex tetrakis(μ₂-o-nitrobenzoato-O,O′)-diaqua-copper(II) 1–Cu₂(2-NO₂C₆H₄COO)₄(H₂O)₂ are reported. These two structures have been solved and there is substantial evidence of a nonaquo structure Cu₂(2-NO₂C₆H₄COO)₄ that precipitated from the same aqueous solution. Diaquo form 1 crystallizes as emerald green prisms of varying sizes and is the thermodynamically stable form. Crystal data: M_r = 827.56, monoclinic, a = 7.534(2), b = 13.411(4), c = 15.404(4) Å, β = 97.101(5)°, V = 1544.5(7) ų, T = 295(2) K, space group P2₁/n, Z = 2, μ = 1.47 mm⁻¹, D_x = 1.780 Mg m⁻³, λ(Mo Kα) = 0.71073 Å, F(000) = 836, final R_[int] = 0.0397 for 3725 reflections used in the refinement. Diaquo form 2 crystallizes in two habits, plates and diamondoids, and is the kinetically stable form. The crystals are usually thinner and consequently lighter shaded than form 1. Crystal data: M_r = 827.56, monoclinic, a = 26.561(3), b = 7.4708(9), c = 16.652(2) Å, β = 106.510(3)°, V = 3168.1(6) ų, T = 293(2) K, space group P2₁/c, Z = 4, μ = 1.434 mm⁻¹, D_x = 1.735 Mg m⁻³, λ(Mo Kα) = 0.71073 Å, F(000) = 1672, final R_[int] = 0.0539 for 7828 reflections used in the refinement.