Two decades (1997–2017) of blended satellite-retrieved phytoplankton chlorophyll a concentration (Chl-a) data were used to assess the interannual variation and recent trends of phytoplankton biomass in Arctic and subarctic ocean (ASAO) regions. Satellite and reanalysis data for geophysical variables were used to assess the probable environmental factors responsible for the observed Chl-a changes. Aggregate Chl-a in the entire ASAO increased steadily from 1997 and then began to decline in the early 2010s. This decline reflects Chl-a declines in most ASAO regions, but the responsible factors and the year in which the trend reversal occurred varied greatly from region to region. In open ocean regions, large-scale climate variability might underlie the Chl-a decline, whereas, in semi-enclosed coastal waters and embayments, the underlying factors might be local or regional (including land–ocean interactions). Therefore, although sea surface temperatures in the ASAO continued to increase steadily, the decline in aggregate Chl-a in the ASAO is unlikely to be connected to global warming.