The synthesis, crystal structures, photoluminescence, and magnetic properties of the melilite-type oxysulfide Sr2MnGe2S6O were investigated. This compound crystallizes in the melilite structure with space group P4 ($) over bar2(1)m, in which two kinds of anions, S2- and O2-, occupy different crystallographic sites in an ordered manner. The temperature dependence of the magnetic susceptibility of Sr2MnGe2S6O shows a broad peak due to a two-dimensional magnetic interaction between Mn ions in the ab plane. The specific heat data show that this compound has an antiferromagnetic transition temperature (T-N = 15.5 K) that is much higher than that of the oxide analogue Sr2MnGe2O7 (T-N = 4.4 K). DFT calculations showed that the magnetic interaction is enhanced by covalency in the Mn-s bonding.