The AnMnSe 3 ( An=Th, U) compounds were synthesized from high-temperature solid-state reactions of the constituent elements at 1223 and 1273 K, respectively. Both compounds are isostructural with UFeS 3 and crystallize in the space group Cmcm of the orthorhombic system with four formula units in a cell. Cell constants (Å) at 153 K are: ThMnSe 3, 4.0304(4), 12.795(1), 9.2883(9); UMnSe 3, 3.931(5), 12.705(14), 9.148(10). The structure comprises layers of MnSe 6 octahedra that alternate with layers of AnSe 8 bicapped trigonal prisms along the b-axis. Because there are no Se-Se bonds in the structure of AnMnSe 3 the formal oxidation states of An/Mn/Se are 4+/2+/2-. UMnSe 3 is a ferromagnet with TC=62 K.