Estuaries are coastal areas that have a fairly high fertility rate. This region can be said to be fertile because it is influenced by such physical factors as temperature, turbidity, light, and current as well as such chemical factors as salinity, dissolved oxygen, pH, and nutrient content. The estuary of the Banjar Kemuning River is a coastal area with a lot of human activities, and it is close to the mainland, causing the change in the nutrient content in the waters. The nutrient content in the waters is closely related to the phytoplankton abundance, as the higher the nutrient content in the waters, the greater the phytoplankton abundance and the chlorophyll-a content. The level of chlorophyll-a content in the waters can indirectly be used as an indicator of the phytoplankton abundance, which can determine the level of fertility of these waters. The purpose of this study was to determine the relationship between the phytoplankton abundance and the chlorophyll-a content in the estuary of the Banjar Kemuning River. This study is observational research with data analysis using linear regression, simple correlation (Pearson), and ANOVA. In addition, the data collection was carried out by purposive random sampling at predetermined points of stations. The results of this study indicated that the phytoplankton abundance and the chlorophyll-a content at the estuary of Banjar Kemuning River during different months were significantly different (P < 0.05). The average phytoplankton abundance during the study was 19,256 cells/L, 19,044 cells/L, and 22,613 cells/L, while the average chlorophyll-a content during the study was 0.958 µg/L, 0.998 µg/L, and 1.167 µg/L. The conclusion was drawn that there is a difference between the phytoplankton abundance and the chlorophyll-a content each month and classified into high correlation.