The structure of argentopyrite, AgFe2S3, was determined for the first time with single-crystal X-ray diffraction. In contrast to the previously reported orthorhombic symmetry, our data show that argentopyrite is monoclinic with space group P1121/n (non-standard setting) and unit-cell parameters a = 6.6902(2), b = 11.4497(4), c = 6.4525(2) Å, γ = 90.2420(8)°, and V = 494.26(3) Å3. Similar to cubanite (CuFe2S3), the structure of argentopyrite is also based on approximately hexagonal close-packed S atoms, with cations ordered over one half of the tetrahedral sites, forming corner-shared AgS4 and FeS4 tetrahedral sheets parallel to (001). The two structures differ chiefly in the linkage between the two adjacent tetrahedral sheets and the ordering patterns of cations within a tetrahedral sheet. Topologically, the structure of argentopyrite can be obtained by a displacement of a tetrahedral sheet in the cubanite structure along the (a/2 + b/6) direction relative to the sheet beneath, giving rise to a cluster of four edge-shared FeS4 tetrahedra in argentopyrite, as compared to two in cubanite. There are two distinct Fe sites (Fe1 and Fe2) in argentopyrite, rather than only one, as in other MFe2S3 sulfide minerals (M = monovalent cations). Together with published Mössbauer data, we suggest that there exists some degree of Fe2+-Fe3+ order-disorder in argentopyrite, with Fe2+ favoring the more distorted Fe2 tetrahedral site. Argentopyrite appears to possess all the features proposed by Putnis (1977) for a high-temperature ordered form of cubanite.