Phytoplankton are tiny creatures floating on water mass. Inside their cells, there is the pigment chlorophyll-a, a kind of substance used in photosynthesis. As a result, it is essential for the ecosystem of water sources because it is considered as the primary producer. This research aims to analyze the content and distribution of chlorophyll-a on the sea surface. It uses data from the Aqua/MODIS Satellite in a case study of the upper Gulf of Thailand in 2020. The SeaWiFS Data Analysis System (SeaDAS) program analyzed the obtained data. The results revealed that the content and distribution of chlorophyll-a on the sea surface in the area of the upper Gulf of Thailand were based on seasons and the influence of monsoons. It was found that the content of chlorophyll-a was highest in the southwest monsoon period (May-October) and lowest in the northeast monsoon period (November-April). The content of chlorophyll-a was highest in October with a mean of 3.148 mg/m³, followed by November, and it was lowest in February with a mean of 1.514 mg/m³. Moreover, it was also found that, if the sea surface temperature was higher than 30°C, phytoplankton exhibited proper growth and propagation. As a result, the content and distribution of chlorophyll-a were consistent with seasonal changes and weather variability.