Chilika Lake in Odisha, India, is the largest brackish water lake in the country and the second-largest coastal lagoon globally. Its dynamic ecosystem, shaped by tidal actions, wave dynamics and freshwater inflows from the Mahanadi River, experiences seasonal water-spread variations during the monsoon. Chilika is a biodiversity hotspot, hosting various taxa, including foraminifera—marine protozoans essential for biostratigraphy, palaeoecology, and palaeoenvironmental studies. To assess foraminiferal diversity and distribution; a 20 cm sediment core and 15 surface sediment samples (100 cm3 each) were collected during the pre-monsoon season, following standard sampling and processing procedures. Analysis revealed eight species, comprising both hyaline (calcareous) and agglutinated forms: Ammonia beccarii, A. tepida, Haynesina depressula, H. germanica, Trochammina inflata, Cribroelphidium poeynum, C. hispidulum, and Elphidium advenum. Statistical analyses, including TFN and Murray’s Ternary Diagram, showed a dominance of calcareous hyaline forms in both core and surface samples. Core diversity peaked at 13 and 19 cm depths, while surface diversity was highest at station S4. TFN ranged from 0 to 1011 per gram in surface samples and 0 to 23 per cm in core samples. Fisher’s alpha index (α = 0–1) indicated low assemblage diversity, reflecting ecological changes linked to historical lagoon conditions. Periods of sea mouth closure led to sediment deposition and fresher lagoon conditions, resulting in lower foraminiferal abundance. In contrast, brackish conditions were restored after artificial and cyclone-induced sea mouth openings, possibly influenced by human activities. These findings provide a crucial ecological baseline for understanding Chilika Lake’s dynamic lagoon environment.