This study assesses the accuracy of lake surface temperature (LST) retrievals for Lake Ladoga based on MODIS radiometers aboard the Aqua and Terra satellites during the warm season (May–October) from 2003 to 2013. The validation employed 245 in situ surface layer measurements collected by the Institute of Limnology, Russian Academy of Sciences, together with 62 instantaneous satellite L2 fields generated using the NASA algorithm with a spatial resolution of 1 km. A comparison between satellite and in situ observations revealed a pronounced linear relationship with high coefficients of determination (R2 > 0.9), confirming the reliability of MODIS products for evaluating the spatio-temporal variability of lake surface temperature. At the same time, a systematic positive bias of satellite-derived values relative to ship-based measurements was identified, reaching up to 0.5 °C. A detailed error analysis demonstrated that MODIS/Aqua generally outperformed MODIS/Terra in terms of mean absolute error (0.73 °C vs. 0.85 °C), although it was characterized by pronounced seasonal variability: in May–June, Aqua exhibited substantial positive bias with low error dispersion, whereas in July–October the systematic bias decreased while variance increased markedly. By contrast, MODIS/Terra showed temporal stability, maintaining consistent bias and standard deviation values throughout the entire observation period, making its records more predictable and reliable for constructing long-term climate series. These findings confirm the fundamental applicability of both radiometers for monitoring the thermal state of Lake Ladoga, while highlighting the need to account for instrument-specific error characteristics when analyzing seasonal thermal dynamics and developing climate-quality datasets.