The study was conducted to assess the quality of surface coastal water in Tien Giang province based on benthic composition. The zoobenthic samples were collected at 9 monitoring stations with the frequency of five times per year. Biodiversity indices including Shannon-Weiner biodiversity index (H') and Barger-Parker dominance index (D) were applied to assess coastal water quality. A total of 52 species belonging to 7 benthic classes were found, of which the Polychaeta class was the most abundant species. The composition and density of benthic species between the rainy and dry seasons were significantly different. The number of benthic species varied from 4 species (dry season) to 48 species (rainy season) with an average density of 25 individuals/m² and 114 individuals/m², respectively. In the dry season, Nereis sp. predominated while Dotilla wichnanni predominated in the rainy season. The values of H' showed that the composition of benthic species varied from poor diversity in the dry season to good in the rainy season, reflecting the quality of the water from heavy pollution (in the dry season) to β-mesosaprobic pollution (rainy season). The dominance index (D) showed that the benthic species was very unstable in the dry season to very stable in the rainy season, reflecting the variations of water quality from α-mesosaprobic pollution (dry season) to less pollution (rainy season). The findings presented that seasonal fluctuation of coastal water quality could be related to the variations of zoobenthic composition and biodiversity indices.