Abstract The subarctic northeast Pacific (SNEP) is a high‐nutrient, low‐chlorophyll region where primary productivity is limited by bioavailable iron during the spring through autumn, and by light limitation during winter. Here, we investigate the spatio‐temporal distribution and drivers of SNEP surface phytoplankton biomass and community composition in the winters of 2019 and 2020 using in situ environmental data, chemotaxonomic analysis of phytoplankton pigment samples, and Sentinel‐3A Ocean Land Color Instrument imagery. The utilized satellite‐based algorithm showed promise replicating the expected trends of: (a) homogenous phytoplankton communities dominated by haptophytes, green algae, and pelagophytes in highly mixed light‐limited oceanic waters and; (b) increased diatoms in coastal Haida Gwaii waters with reduced mixed‐layer depth (MLD) and salinity. Unexpectedly, increases in cryptophytes were observed in the northern extents of the SNEP, which coincided with winter marine heatwave driven reductions in MLDs and also the presence of a mesoscale eddy. This finding highlights a deviation from expected homogeneous phytoplankton conditions, which may be systematically missed by spatially and temporally constrained in situ sampling. The further advancement and deployment of the satellite‐based algorithm could significantly expand the understanding of winter phytoplankton dynamics in the SNEP, a critical period for Pacific salmon survival, improving the understanding of trophic linkages and match/mismatch dynamics, and contributing to improve the forecasting of salmon returns. Plain Language Summary Phytoplankton are the foundation of the marine food web, significantly regulating biogeochemical processes and contributing to about half of the global primary productivity. Therefore, monitoring phytoplankton community composition is critical for understanding the ocean's role in climate change and fisheries. Satellite observations are more cost‐effective than ship‐based observations, with a much higher spatial and temporal resolution. Here, we combined field‐collected samples of phytoplankton pigments and satellite imagery to estimate phytoplankton composition in the winter of 2019 and 2020 across the northeast Pacific. The satellite outputs aligned well with field data, giving confidence to this method. Generally low biomass and consistent phytoplankton communities, dominated by small flagellates called haptophytes, were observed across varying environmental conditions over these iron‐limited open ocean regions. Diatoms were more prevalent near Haida Gwaii, where iron availability was likely higher and mixing was less pronounced, likely increasing light availability for the growth of these large species. Additionally, cryptophytes increased in the northern study area, possibly due to the influence of a marine heatwave and a mesoscale eddy. This study showcases the potential application of satellite imagery for investigating phytoplankton community composition over the subarctic northeast Pacific, which provides habitats for various commercially important pelagic fisheries, including Pacific salmon. Key Points Satellite ocean color and in situ data characterized winter surface phytoplankton community composition across the northeast Pacific Increased cryptophyte biomass was observed during marine heatwave conditions and the presence of an eddy The employed method expands the limited winter phytoplankton composition data from an undersampled region vital for Pacific salmon survival