Abstract. Ynezidinium malloyi gen. nov., sp. nov. shows paratabulation details that indicate it belongs to the Family Gonyaulacaceae, subfamily Leptodiniodeae, but which, in combination, distinguish it from previously described genera. The paratabulation details include: (1) pentagonal sixth precingular (li) in contact with both fourth and first apicals (A and lu); (2) elongate and subparallel fourth and first apicals; (3) straight to slightly sigmoidal sulcal region; (4) lack of small intercalary (K) paraplates; and (5) Q/B preapical arrangement. Other species herein assigned to Ynezidinium include: Ynezidinium brevisulcatum (Michoux, 1985) comb. nov., Y. latolineatum (Yun, 1981) comb. nov., Y. pentahedrias (Damassa, 1979) comb. nov. and Y. waipawaense (Wilson, 1988) comb. nov.