The objective of this study was to determine the seasonal fluctuation of water quality elements and zooplankton composition in the Hau river, Vietnam. The study was conducted four times a year in An Giang, Can Tho, and Soc Trang provinces. Sampling was conducted at 29 sites, including 14 sites on the main river and 15 sites on its tributaries. The mean temperature, pH, dissolved oxygen (DO) and biological oxygen demand (BOD) concentrations at sampling locations did not change significantly between the dry and rainy seasons. Nitrate (NO3 - ) and phosphate (PO4 3- ) concentrations in the dry season were higher than those in the rainy season. A total of 148 zooplankton species was recorded. Among them, Rotifera was the most abundant group, with 52 species, followed by Protozoa with 48 species, and the others ranging from 10 to 22 species. The species composition of zooplankton in the tributaries was higher than in the main river. At the same time, species composition in the rainy season was higher than in the dry season. The densities of Copepoda, Rotifera, Cladocera, and Copepod’s nauplii were found to have significant negative correlations with NO3 - concentrations (p < 0.05). However, close correlations were not recorded between these organisms and concentrations of chemical oxygen demand (COD) and total phosphorus (TP) in the Hau River. The Shannon-Wiener diversity index (H’) fluctuated from 1.6 to 3.2 and Margalef’s richness index (D) fluctuated from 0.9 to 4.6 indicating that the pollution level of the study area was from light to β-moderate pollution. The results of this study provided a reference for studying the composition and distribution of seasonal zooplankton communities, water quality characteristics, and pollution levels in the Hau River.