This paper presents the trophic state index (TSI) and water physico-chemical characteristics of a shallow tropical reservoir located in southeast Brazil. It describes a study held over a hydrological and diurnal period, which enabled to map the variability of several physico-chemical variables due to regional climate and to compare the results with the TSI in winter and summer. The thermal behavior of the water column is typical of a polymictic reservoir. Low dissolved oxygen levels have been measured, reaching 0.07 mg L−1 in the deepest part of the reservoir as a direct consequence of the increased nutrient levels. The average pH was slightly acid (6.71) especially at the reservoir bottom and did not show significant changes during the monitoring period. The electrical conductivity varied according to seasons, i.e., it was higher in the rainy period compared to dry season, averaging 138 and 84.06 µS/cm, respectively. According to the TSI, the reservoir has been classified as mesotrophic and eutrophic in the dry and rainy season, respectively.