Phase pure Ag2MnSnS4 was synthesized from the elements using standard high-temperature solid-state methods. Its crystal structure was solved from single-crystal X-ray diffraction data collected from a pseudo-merohedrally twinned crystal as well as by Rietveld refinement of X-ray powder diffraction data. Ag2MnSnS4 crystallizes in the monoclinic space group Pc (no. 7) with the unit cell parameters a = 6.651(1), b = 6.943(1), c = 10.536(2) angstrom, beta = 129.15(1)degrees, V = 337.3(1) angstrom(3), and Z = 2. The tetrahedraly compound crystallizes in a superstructure of the wurtzite type and the tetrahedra volumes are in good agreement with the model for diamond-related compounds derived from the wurtzite structure type. The red semiconductor Ag2MnSnS4 has an optical bandgap of E-g = 2.0 eV and is stable up to its peritectic decomposition temperature of approximately 700 degrees C. Ag2MnSnS4 is a Curie-Weiss paramagnet with an experimental magnetic moment of mu(exp) = 5.4(1) mu(B) per manganese atom. Antiferromagnetic ordering is detected at a Neel temperature of T-N = 8.8(1) K. Sn-119 Mo ss bauer spectra at 78 K underline the single tetrahedrally coordinated Sn-IV site (delta = 1.34(1) mm center dot s(-1)). The 6 K spectrum (magnetically ordered state) reveals a small transferred magnetic hyperfine field of 1.02(1) T.