Coccolithophore is one type of phytoplankton that forms calcium carbonate plates called coccoliths, and is relatively diverse with about 200 species in the worldʼs oceans. Coccoliths accumulate in massive calcareous deposits on the sea floor that serve as a sedimentary buffer of ocean chemistry and are also a major long-term carbon storage that has a significant impact on the global carbon cycle and climate. Coccolithophores are also a major producer of dimethyl sulfide (DMS) that is the dominant precursor for cloud condensation nuclei in the marine atmosphere; DMS emissions decrease solar radiation due to increasing cloud cover. Coccolithophores play an important role in biogeochemical cycles in the ocean by influencing the oceanic uptake of atmospheric carbon dioxide, and producing calcium carbonate sediments and DMS. Coccolithophores often bloom on massive scales. Around the Japan archipelago, coccolithophore blooms have also been reported; a Gephyrocapsa oceanica bloom in Tokyo Bay and Sagami Bay (1995) and Hakata Bay (2004, 2007, 2008), and a Emiliania huxleyi bloom in Suruga Bay (2007). In mid-May 2020, a massive bloom of coccolithophores were observed in Sagami Bay, Japan, possibly for the first time in 25 years. The bloom was initially observed by the JAXA, GCOM-C ocean color remote sensing satellite “Shikisai” capturing a vast emerald green reflectance image in the bay. Researchers who received the report immediately conducted in situ observations, and found that coccolithophores caused the massive bloom. Coccolithophore is a common yet elusive group in the waters around Japan, and there has only been one report of a bloom observation in Sagami Bay. This study shows the characteristics of the coccolithophore bloom observed in Sagami Bay in 2020 and also reports on the marine environment during the bloom. Water temperature, salinity, nutrients, chlorophyll a, and cell density were collected from field samples. Satellite ocean color data was provided by “Shikisai”. Satellite images on May 13 and 17 in 2020 show that the bloom spread counterclockwise due to counterclockwise coastal currents in Sagami Bay. The species, Gephyrocapsa oceanica, was confirmed using light microscopy and a scanning electron microscope, and was the same species that occurred in 1995. Chlorophyll a concentration and cell density at the surface were 1.2 μg L-1 and 9.0×103 cells mL-1, respectively. NO2- and NO3-concentrations at the surface during the bloom were 0.10 μmol L-1 and 1.83 μmol L-1, respectively, which were relatively higher than previous years. The results suggest that one of the causes of coccolithophore bloom is the relatively high concentration of inorganic nitrogen. Previous studies of prokaryotic and eukaryotic community structure in the coastal areas of Sagami Bay using genetic analysis have reported occurrence of haptophytes and the occurrence of coccolithophores throughout the year. These results suggest that coccolithophores are frequently present in Sagami Bay and form massive blooms in a short time period if environmental conditions are favorable. Further investigation including culture experiments are needed to resolve the details and specific causes.