A quasi-two-dimensional manganese-based compound ThMnSbN is synthesized by the solid-state reaction method. Structural refinement based on X-ray powder diffraction shows that the compound structure belongs to the P4/nmm space group. The lattice parameters are a = 4.1731 Å and c = 9.5160 Å. Electrical transport measurements show that the resistivity of the compound is the lowest in the Mn-based family. When cooling it, its resistivity rises slowly and shows a shoulder-like anomaly at 16 K. Also, the magnetic susceptibility exhibits an anomaly at the very same temperature. Though the specific heat data indicate the inexistence of transition-induced anomaly, the electron specific heat coefficient of γ = 19.7 mJ·mol^–1·K^–2 is derived by fitting the low-temperature C-T curve. This γ value is much higher than those of the isostructural manganese-based compounds. Thus, the specific heat is consistent with the low resistivity, implying a considerable electronic density of states near the Fermi surface for ThMnSbN. By comparing the crystal structure for a group of ZrCuSiAs-type compounds, various chemical pressure effects of the fluorite-type Th₂N₂ layer on the conducting layer in different compounds are discussed.