We report the realization of a metallic flat band in the marcasite compound MnSb2. The material was synthesized under high pressure, and its crystal structure was established by X-ray diffraction. Electrical and thermoelectric transport measurements reveal signatures of a flat band near the Fermi level that persist across three structural, magnetic, and electronic transitions. These observations are supported by extensive density-functional-theory calculations. Related marcasite diantimonides, including FeSb2 and CrSb2, also host flat bands, but these lie far from the Fermi level and therefore do not produce a metallic flat-band state. MnSb2 therefore emerges as the only known metallic marcasite antimonide in which a flat band lies in the vicinity of the the Fermi level.