Ongoing climate change is expected to transform ecosystems worldwide. Time series of remotely sensed data are now of sufficient length to begin to assess change in the ocean at large spatial and temporal scales. This study focused on changes in the phytoplankton phenology and composition in the subarctic Pacific Ocean, winter residence region for Pacific salmonids. A time series of satellite phytoplankton phenology metrics and phytoplankton functional groups between 2002 and 2022 were analyzed. Additionally, potential drivers of change were determined among the essential environmental factors and climate indices. Using changepoint analysis, a decrease in the total bloom length was revealed in recent years in all bioregions except for the waters surrounding the Kamchatka Peninsula. Moreover, a decreasing trend in the diatom-to-dinoflagellate Chl-a and the diatom-to-small algae Chl-a, consisting of haptophytes, pelagophytes, green algae, and cyanobacteria, was observed in the Gulf of Alaska. A sharp decline was particularly pronounced after 2018, which probably stemmed from a combination of the weaker currents forming the North Pacific Gyre Oscillation (NPGO) and recurring marine heat waves after 2014. It is uncertain yet whether the decline of the diatom group is temporary or marks the beginning of a long-term shift in the phytoplankton community structure in the subarctic Pacific. The following years will likely bring the answers.