The Thalassiosira trifulta group has few morphological features as observed in the LM (light microscope) that allow its evolution to be traced. This study and our previous studies described this diatom group in detail based on LM and SEM (scanning electron microscope) observations, and distinguished three subgroups, the Thalassiosira frenguelliopsis subgroup, the Thalassiosira oestrupii subgroup, and the Thalassiosira bipora subgroup. Here we define the stratigraphic ranges of the species which were defined in the T. trifulta group by integrating LM and SEM observations on samples from deep-sea cores in the northwestern Pacific Ocean. The detailed SEM observations of fultoportulae (both operculate and trifultate) and small pore-like structure in the valve centre (occluded areola in our previous studies) provide a reliable classification based on the phylogenetic evolution. Thalassiosira bipora f. marginata n. f., Thalassiosira bipora f. minima n. f., Thalassiosira bipora f. prima n. f., Thalassiosira oestrupii f. vetus n. f., Thalassiosira praeoestrupii f. juvenis n. f., and Thalassiosira mesopora n. sp. are described and assigned to the T. trifulta group. Four events concerning the differentiation and evolution of this group during the intervals of 5.7-5.4 Ma, 5.3-4.8 Ma, 3.4-3.2 Ma, and 2.5-2.2 Ma are defined and discussed.