A detailed study on foraminiferal assemblage, textural characteristics and clay mineralogy have been carried out for 20 surface and two core samples recovered from Yedayanthittu lagoon. It is situated 3km north of Kaliveli Lake which is one of the largest brackish water lake in the peninsular India. The lagoonalforaminiferal assemblage comprises rotalids, miliolids and agglutinated formsof which some forms are specific to littoral to sublittoral zones of sea. Ammonia beccarii, A. tepida, A. parkinsoniana, Rotalidium annectans and Elphidium advenumare copious forms of rotalids, Ammobaculites agglutinans, Ammobaculites persicus and Ammobaculites exiguus are abundant agglutinated forms, and in miliolids Quinqueloculina seminula, Miliammina fusca and Adelosina longirostra are the dominant ones. Most of the sediments are characterized by medium sand, moderately to poorly sorted and negatively to positively skewed. Temporal variation of textural characters of the sediment reveals that the energy condition was moderate, and below 110 cm depth the lagoon has experienced either erosion or non-deposition. Kaolinite is the predominant clay mineral in the area followed by Illite, Smectite and Chlorite. Kaolinite would have been formed by chemical weathering of pre-existing rocks of adjacent upland areas under tropical humid climatic condition.