The new ternary nitride Ca6MnN5 has been synthesized by the solid state reaction of Ca3N2 with Mn4N and characterized by powder X-ray diffraction. Ca6MnN5 crystallizes in the hexagonal space group P63/mcm (a 6.2269(1) Angstrom, c = 12.3122(1) Angstrom, Z = 2) and is isostructural with Ca6FeN5 and Ca6GaN5. Manganese(III) forms trigonal planar [MnN3](6-) units analogous to carbonate anions, which lie in planes parallel to the c axis, sandwiched between layers of Ca-N polyhedra. Calcium is coordinated to five nitrogens in a distorted square pyramidal geometry. Interestingly, at room temperature the material is diamagnetic.