The Quan Son Reservoir is the primary source of water supply for agricultural cultivation, farming, and aquaculture in My Duc district, Hanoi. Therefore, it is indispensable to assess and frequently monitor the reservoir's water quality. This study aims to determine the eutrophication status of Quan Son Reservoir and the spatial and temporal changes of the Trophic State Index (TSI) based on in situ datasets of Chlorophyll–a (Chla) concentration, total phosphorus (TP) concentration, and water transparency (SD) obtained from 78 points in four surveys from November 2021 to October 2022. The results show that Quan Son Reservoir is highly eutrophic with TSI value of > 60 at all measurement points. TSI values vary in space and time, depending on the water accumulation process of the reservoir as well as surrounding human activities. The results help better understand the state and trends of eutrophication in Quan Son Reservoir, thereby proposing solutions to respond to the reservoir’s environmental issues proactively.