The structure of the title compound, [Mn(tpy)2](S4O6)*3H2O (tpy is 2,2':6',2"-terpyridine, C15H11N3), consists of monomeric [Mn(tpy)2]2+ units embedded in a complex anionic network made up of tetrathionate ions and hydration water molecules connected via a complex hydrogen-bonding scheme.