Skeletonema species are among the most important components of phytoplankton communities in the temperate and tropical Pacific Ocean. Data on Skeletonema species diversity and abundance in the northeastern Pacific are fragmentary. The long-term observations (1996–2019) of the phytoplankton from the coastal waters of the northwestern Sea of Japan resulted in five Skeletonema species being recorded: Skeletonema costatum , S. dohrnii , S. japonicum , S. marinoi and S. menzelii. For each of the species, a morphological description is provided based on electron microscopy. Ecological factors influencing the distribution of the species and their occurrence under certain environmental parameters are discussed. One of the species, S. menzelii , is a new record for Russia. Skeletonema costatum was confirmed to occur in the study area. Seasonal and interannual changes in the abundance of Skeletonema species were studied. The concentrations of the species varied from 20 cells l −1 to 12,700,000 cells l −1 . From 1996 to 2018 there was a downward trend in the numbers of Skeletonema . Skeletonema japonicum was noted for the first time to bloom. Skeletonema dohrnii formed summer blooms; S. japonicum bloomed in the autumn. No Skeletonema blooms were detected in the winter months.