The synthesis, crystal structure and magnetic properties of the brown oxide selenide Sr2Mn2O4Se are described. The compound crystallises in space group Pnma with room temperature lattice parameters a = 9.4825(1), b = 10.3356(1) and c = 6.23895(8) Å. The structure contains Mn3+ ions in highly distended MnO4Se2 octahedra with selenide ions in the axial positions. These octahedra share two of their faces with neighbouring octahedra with the result that all the manganese and oxide ions are involved in the formation of undulating [MnO4/2]– chains directed along the a axis, which form corrugated chains along the c axis by sharing the selenide vertices of the octahedra. These chains are isolated from one another along the b axis. Magnetic susceptibility measurements show that the compound does not obey the Curie-Weiss law below room temperature and there is a magnetic transition at 160(5) K associated with antiferromagnetic ordering of the Mn3+ moments. The magnetic structure measured at 5 K is described by a k = (0, 0, 0) propagation vector and nearest neighbour moments are coupled antiferromagnetically.