The dinophyte family Amphidomataceae, comprising the genera Azadinium and Amphidoma, includes several species known to produce azaspiracids (AZAs), a group of lipophilic marine biotoxins associated with shellfish poisoning. Despite their ecological relevance and the increasing number of newly described species in recent years, their diversity is likely still underestimated, and knowledge of their global distribution and biogeography remains limited. Based on detailed morphological and phylogenetic analyses of strains from Korea and Japan, we describe Azadinium fusiforme sp. nov., a taxon that corresponds to the phylogenetic position of a strain previously reported from Japan. Light and scanning electron microscopy revealed that Az. fusiforme possesses a unique combination of morphological traits that distinguish it from its congeners. Notably, the ventral pore is located on the left side of the first apical plate, the pyrenoid is consistently positioned in the posteriormost part of the cell, the nucleus is elongated and centrally located, and pronounced thickenings occur along the sutures between the lateral and dorsal apical plates. Molecular data place Az. fusiforme in a well-supported, distinct clade within Azadinium, clearly supporting its designation as a new species. Mass spectrometric analyses showed no evidence of AZA production in any of the Az. fusiforme strains. This study contributes to a more fundamental understanding of the species diversity, distribution, and potential toxicity of Amphidomataceae in the Asia-Pacific region.