Abstract: Nutrient stoichiometry, such as carbon (C): nitrogen (N): phosphorus (P), is a powerful indicator of biogeochemical processes in lakes. However, relatively little is known about how the nutrient stoichiometry of river-connected lakes dynamically responds to driving factors across different hydrological seasons. Here, we selected a 15-yr water-quality dataset (2007–2021) to examine spatiotemporal variations in the stoichiometry of Dongting Lake of China and to explore the relationship between the lake’s stoichiometric characteristics and driving variables. Our analysis suggested that the N:P ratios of the lake were between those of the three channels and four tributaries, with a decreasing trend from 6.01 in 2007 to 1.32 in 2021; whereas the C:N ratios in the four tributaries, three channels, and lake exhibited increasing trends during the study period. Tributary stoichiometric ratios were significantly positively correlated with those of Dongting Lake, demonstrating that upstream river inputs exert strong control over lake elemental stoichiometry along the tributary-to-lake continuum. Nutrient stoichiometric ratios showed significant spatial heterogeneity between the flood and dry seasons from the tributaries to Dongting Lake. The stoichiometric ratios in the tributaries exhibited significant positive correlations with the N:P and C:N ratios of Dongting Lake. Increasing trends were detected in water level (WL), which increased from 24.58 m in 2007 to 25. 46 m in 2021. There were strong relationships between the C:P ratios and WL, temperature, and water discharge (WD) across Dongting Lake (P < 0.05). Significant positive relationships were detected between the C:N ratios and WL, temperature, and WD, explaining 15.15%, 12.34%, and 14.24% of the variability, respectively. The stoichiometry of Dongting Lake in different hydrological seasons clearly differentiated the flood and dry seasons, owing to the effect of variations in hydrological conditions within a year. Our results indicated tributary inputs and lake hydrology jointly modulate lake stoichiometry, improving mechanistic insights into biogeochemical cycling of river-connected lakes.