Well-preserved marine diatoms are documented for the first time from authigenic carbonate rocks induced by cold methane (hydrocarbon) seepage in the Omagari Formation (latest Santonian to earliest Campanian in age, Late Cretaceous Epoch; around 83.6 Ma) of the upper part of the Yezo Group in the Teshio-Nakagawa area, northern Hokkaido (northern Japan). The diatom flora is rich in species of Hemiaulus and Triceratium, associated with a few other extinct diatom genera. An araphid genus (Sceptroneis) was also observed; this is one of the earliest fossil records of “pennate” (Bacillariophyceae) diatoms. Although valve ultrastructures have been mostly dissolved, the preservation of these diatoms is much better than that in the few previous reports of Cretaceous siliceous photosynthetic organisms from Japan and adjacent regions in the northwestern Pacific Ocean. Because of their common generic composition in the mid- to high-latitude regions in both hemispheres, diatoms are suggested to have experienced a global radiation by early Campanian time. Thus, our diatom records provide insights into the Late Cretaceous floral adaptive radiation around the northwestern Pacific margin, where the radiation history of diatoms is yet unclear. As Gladius antiquus was confirmed but Basilicostephanus species were absent from the present material, the flora from the Teshio-Nakagawa area is tentatively regarded as belonging to the G. antiquus Concurrent Range Zone, an interval extending from an undetermined Late Cretaceous Epoch to the latest Santonian Age. However, this floral correlation is inaccurate because our materials are latest Santonian to earliest Campanian in age, as dated by the molluscan (ammonoids and inoceramids) biostratigraphy. Hence, further research is required to clarify the sensitivity of different chronological proxies and the stratigraphic ranges of age-diagnostic diatoms in different geographic provinces.