Coastal polynyas are critical polar marine systems that benefit from solar radiation in summer, triggering early phytoplankton blooms. However, phytoplankton dynamics within the coastal polynya in the Ross Sea remain understudied due to harsh polar conditions. This study delineated the coastal polynya using sea ice concentration (SIC) data and investigated spatio-temporal variations in chlorophyll-a (Chl) concentration from November 2003 to March 2025. Results showed that Chl formed a long belt along the Ross Ice Shelf, extending northward with decreasing concentration. Phytoplankton blooms initiated in November, expanded through December and January, and declined by February and March, with monthly average Chl ranging from 0.78 mg/m³ to 2.15 mg/m³. Seasonal Chl variations aligned with photosynthetically active radiation (PAR) and sea surface temperature (SST), but were inversely related to SIC and wind speed (Wspeed). A notable increase in Chl (0.03 mg/m³ yr⁻¹) was observed after 2014, alongside increasing PAR (0.23 einstein/m²/d yr⁻¹), which was identified as the dominant driver. Spatial patterns illustrated that high PAR (35–40 einstein/m²/d) and low SIC (0–5%) promoted bloom development. Moderate SST (−1.2°C to −0.6°C) and offshore winds (Wspeed: 5–7 m/s) influenced bloom intensity and extent, while wind direction shaped Chl dispersion. These findings enhance our understanding of phytoplankton dynamics and potential climate responses in this key polar region.