A new version of Trophic State Index for freshwater Himalayan lake (TSIFHL) has been derived from Landsat 8 OLI to determine the aquatic health of the lake ecosystem. TSIFHL is based on chlorophyll-a concentration (CChl-a) which has been retrieved from Landsat 8 OLI data and laboratory measurements using an empirical approach. Further, in-situ measurements have also been taken with Secchi disk depth (ZSD) in a freshwater Himalayan lake (FHL). The derived CChl-a exhibited lower and upper limit of 25.81 µg/L and 207.96 µg/L respectively. The modelled ZSD values ranged between 0.18 m to 0.66 m with an average depth of 0.50 m. The best-fitted regression model, developed for CChl-a with R² = 0.89, exhibited model error of 0.77 µg/L for the Standard Error of Estimate (SEE). The Mean Absolute Percentage Error (MAPE) and Nash–Sutcliffe coefficient (E) values were 5.83 % and 0.98 µg/L respectively. For the ZSD, the best-fitted model showed errors of 0.11 µg/L (SEE), 13.93 % (MAPE), and 0.77 µg/L (E) with R² = 0.84.