We describe Mutabella mirabilis, a new genus and species of Miocene microperforate planktonic foraminifera. The species is known from several drill sites in the tropical Pacific, Indian and Atlantic Oceans (Deep Sea Drilling Project / Ocean Drilling Program Sites 78, 219, 865, 871, 872, 873, 959, and 960). The taxon shows an unusually high level of intraspecific variability in chamber shape and arrangement, aperture position, and test ornament. Some specimens possess supplementary sutural apertures, which may have bulla-like coverings. Others possess a composite tooth-plate reminiscent of certain Cretaceous species. Mutabella mirabilis first evolved in early Miocene Biochron M3, and probably became extinct in middle Miocene Biochron M7. A morphometric study from Site 872 (west Pacific) demonstrates that substantial shape change occurred during its evolution, related to the height of the trochospiral chamber arrangement and degree of involution of the test. A stable isotopic comparison with other coexisting species indicates that M. mirabilis occupied a shallow (mixed layer) planktonic habitat. A strong positive correlation between test size and δ13C indicates that it probably associated with photosynthetic symbionts. Mutabella mirabilis seems to have evolved from the long-ranging species Globigerinita glutinata, with which it fully intergrades in morphology near the bottom of its stratigraphic range.