The management of the ecological integrity of the aquatic ecosystem requires monitoring, analysis, and assessment of the water quality. Thus, this chapter presents the spatiotemporal distribution of basic physicochemical water quality in Tonle Sap Lake based on our recent extensive field survey. The water temperature ranged from 25.6 to 36.9 °C and a region of low water temperature (approx. 29 °C) from 30 to 40 km along the Longitudinal transect. The ranges of pH, electrical conductivity (EC), and total dissolved solids (TDS) were 5.1–9.5, 11–610 μS/cm, and 7–366 mg/L, respectively. Elevated EC and TDS indicated pollution at both ends of the lake. Dissolved oxygen (DO) in Tonle Sap LakeTSL was highly dynamic, varying from anoxic (0.1 mg/L) to highly saturated conditions (16.1 mg/L). Localized anoxia and hypoxia existed, especially in the floating villages and the densely populated northern upstream. Dissolved organic carbon (DOC), particulate organic matter (POM), and oxidation–reduction potential (ORP) varied within the range of 1.6–16.8 mg/L, 0.2–145.7 mg/L, and −212–472 mV, respectively. Turbidity exhibited high seasonal variability, fluctuating by three orders of magnitude with a typical high value during the low-water season.