Ba2CoNbO6 ceramic samples were synthesized by solid-state reaction technique. X-ray photoemission spectroscopy reveals the coexistence of mixed-valent structure of Co2+/Co3+ in the sample. Dielectric measurements reveal that the sample shows colossal dielectric behavior. This behavior is contributed by two thermally activated dielectric relaxations: the low-temperature relaxation is a polaron relaxation resulting from the hopping motions of self-trapped holes; the high-temperature relaxation is a Maxwell–Wagner relaxation caused by the grain-boundary response. Our results indicate a multirelaxation mechanism, i.e., the coupling of the polaronic relaxation and the Maxwell–Wagner relaxations might be a common origin of the colossal dielectric behavior found in different materials.