A new compound, Bi3Mn3O11, with a KSbO3-type structure was prepared using a high-pressure technique. Its structural, chemical, and magnetic properties have been investigated. Bi3Mn3O11 was shown to be a ferrimagnet with rather high T-c despite the random distribution of manganese ions with different oxidation states in one site. Bi3Mn3O11 gradually loses oxygen on heating forming several phases with the same structure but with different magnetic properties. These findings may revive interest in the KSbO3 family of compounds, which is rather adaptive considering the cation variations and oxygen content.