Eutrophication and water pollution have become serious aquatic environmental problems worldwide. In this study, four different water quality indexing methods, the Canadian Council of Ministers of Environment Water Quality Index (CCME WQI), the revised Comprehensive Pollution Index (CPI), the Eutrophication Index (EI), and the Trophic State Index (TSI), were used to investigate water quality and trophic status in the Tri An Reservoir (TAR), Vietnam. Nineteen water variables were monitored bimonthly between February 2018 and December 2019 to calculate these four indices. Although different input data sets were used, all the indices showed similar results in terms of water quality and eutrophic state. The CCME WQI ranged from 32 to 68 (median 36–48), which indicated that the water quality was classified between poor and marginal and was insufficient for domestic purposes. Similarly, the water quality was ranked between slightly polluted and medium polluted based on the revised CPI index, with the median values ranging from 0.72 to 0.94. In the CCME WQI and the CPI, the major parameters that contributed to water pollution in the TAR were turbidity, dissolved oxygen (DO), total suspended solids (TSS), chemical and biological oxygen demand (BOD5), ammonium (NH4+), nitrite (NO2−), iron (Fe), lead (Pb), and coliforms. Regarding the trophic status of water, the EI ranged from 10 to 464, indicating that the reservoir was eutrophic, and the TSI values ranged from 53 to 89, indicating that the water quality was classified between light-eutrophic and hypereutrophic conditions. A temporal variation in water quality was found, with the highest levels of pollution recorded in June during the study period. Our results show that the combination of different water quality indices provides a comprehensive assessment of water quality in the TAR.