ABSTRACT: Trincomalee Bay is situated on the northeast coast of Sri Lanka. This is the first study to report the recent ostracode assemblages in this bay. As a result, at least 36 ostracode taxa were identified from surface sediments in the bay. Many of them are typical tropical-water species that have been reported from inner bays and shallow marine areas around the coast of the Indo-Pacific region. We compared the species composition of ostracodes in the study area with that in adjacent seas. The result clearly showed that ostracode assemblages from Sri Lanka have strong connection with those along the coast of the Indian subcontinent. Moreover we evaluated the bottom environment in Trincomalee Bay on the basis of statistical analyses of ostracode assemblages combined with grain size, and total organic carbon (TOC) and total nitrogen (TN) contents. The results showed that the TOC content is related to grain size and is relatively high in the inner part of Inner Harbor and Koddiyar Bay, where fine-grained sediments are distributed. Four biofacies were recognized based on Q-mode cluster analysis using ostracode data for 36 taxa. Biofacies I is consistent with distributions of fine-grained sediments with moderate TOC contents (0.30%–0.49 %). Biofacies II is collected from relatively deep water areas with sediments of low TOC contents (0.22%–0.41 %) in InnerHarbor. Biofacies III and IVare characterized by euryhaline species and are influenced by fresh water and shallow depth with low TOC contents (0.14%–0.37%) in sediments. The ostracode distribution in Trincomalee Bay is depend on natural environmental factors such as water depth, grain-size, TOC contents of sediments, and water salinity.