Using a hydrothermal reaction, a novel holmium-mercury compound {[Ho(IA)(HIA)₂(H₂O)₂]₂-(Hg₃Br₈)}_n(nHgBr₂)·2nNO₃ (1, HIA is isonicotinc acid) was synthesized and its crystal structure was characterized by single-crystal X-ray diffraction. Compound 1 crystallizes in monoclinic system, space group P2/c with a = 13.0647(6), b = 9.4659(3), c = 26.0832(14) Å, β = 97.522(4)°, V = 3197.9(2) ų, C₃₆H₃₆Br₁₀Hg₄Ho₂N₈O₂₂, M_r = 2863.95, Z = 2, D_c = 2.974 g/cm³, μ(MoKα) = 18.331 mm^–1 and F(000) = 2575. It displays a two-dimensional (2D) layer-like structure. A solid-state photoluminescence experiment revealed that it shows upconversion green emission. The emission peaks should come from the ⁵I₈ → ⁵G₆ and ⁵S₂ → ⁵I₈ characteristic emission of the 4f electrons of the Ho³⁺ ion. Compound 1 has a CIE chromaticity coordinate (0.1774, 0.526). A solid-state UV-visible diffuse reflectance spectrum unveiled that this compound has a wide optical band gap of 3.26 eV.