<p>A phytoplankton bloom is characterized by the rapid and dense proliferation of microscopic algae within aquatic environments. This study investigated the occurrence of blooms, particularly those dominated by the genus <i>Coscinodiscus</i>, in the tropical Patalganga River estuary. The aim was to understand the environmental factors influencing the <i>Coscinodiscus</i> bloom within this complex ecosystem. A total of three <i>Coscinodiscus</i> species, along with 22 associated species from other genera were recorded during the bloom period. The class Coscinodiscophyceae contributed 24%, followed by Bacillariophyceae (32%), Mediophyceae (20%), Dinophyceae (20%) and Cyanophyceae (4%). The genus <i>Coscinodiscus</i> exhibited a cell density ranging from 6.25 × 10³ to 5.43 × 10³ cells/L. The Shannon-Weiner diversity index (H') ranged from 0.30 to 0.54, indicating lower diversity toward the upper part of the estuary, at the upstream freshwater stretch (S6). Diversity indices revealed low species diversity during the bloom period, with <i>Coscinodiscus</i> dominating the phytoplankton community. Cluster analysis and nMDS showed distinct groupings of sampling stations based on species composition. BIO-ENV analysis identified pH, dissolved oxygen, alkalinity and silicate as the most influential parameters on bloom formation and the abundance of other associated species. Canonical correspondence analysis (CCA) indicated that the bloom of <i>Coscinodiscus radiatus</i> was positively correlated with salinity, transparency and alkalinity, while <i>Coscinodiscus wailesii</i> and <i>Coscinodiscus centralis</i> were positively associated with pH, temperature, nitrate, nitrite and chlorophyll-a. The study emphasizes the importance of monitoring phytoplankton blooms and their environmental drivers in the Patalganga River estuary. Understanding these relationships is crucial for the sustainable management and protection of aquatic ecosystems under rising anthropogenic pressures.</p>