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High-altitude Himalayan lakes act as natural storage for environmental evidence related to climate change and environmental factors. A great number of lakes are distributed in the southern slope area of the central Himalayas; however, research concerning the hydrochemical processes of these lakes is still insufficient. Herein, we present a comprehensive study on the water chemistry of the lake waters and the inlet stream waters from Rara Lake in western Nepal based upon samples collected in November 2018. The pH, dissolved oxygen, chlorophyll-a concentration (chl-a), water temperature, electric conductivity (EC) and total dissolved solids (TDS) were measured in situ, and the concentrations of major ions (Ca2+, Mg2+, K+, Na+, Cl−, SO42−, and NO3−) were analyzed in the laboratory. The results revealed that the water in Rara Lake is slightly alkaline, with pH values ranging from 7.6–7.98. The cations, in decreasing order of concentration in the lake water, are Ca2+ >Mg2+ >K+ >Na+ with average concentrations of 20.64 mg·L−1, 11.78 mg·L−1, 1.48 mg·L−1 and 0.72 mg·L−1, respectively; the order and concentrations for the anions is HCO3−>SO42−>Cl−>NO3−, with average concentrations of 122.15 mg·L−1, 2.15 mg·L−1, 0.46 mg·L−1 and 0.55 mg·L−1, respectively. The dominant cation and anion in the lake water are Ca2+ and HCO3−, and they account for 48.14% and 71.8% of the totals, respectively. The range of lake water TDS is from 95 mg·L−1 to 98 mg·L−1, with an average of 96.85 mg·L−1. The high ratio of (Ca2+ + Mg2+) to total cations and the low ratio of (Na+ + K+) to total cations indicate that Rara Lake receives ions from rock weathering, especially from carbonate rocks. Similarly, Gibbs boomerang diagrams and Piper diagrams also support the hydrochemistry of Rara Lake as being dominated by rock-weathering patterns. Likewise, other statistical analysis tools, such as Principal Component Analysis (PCA) and correlation strongly suggest the dominance of weathering of calcium and magnesium bicarbonate rocks as the major source of ions in Rara Lake. However, several traces of anthropogenic inputs into the lake were noticed, and the hypolimnion in the lake appears to be oxygen deficient, which may not be an issue at present but cannot be ignored in the future. [ABSTRACT FROM AUTHOR]