<p>The Qinghai-Xizang Plateau hosts more than 50% of China’s lakes, making the investigation of their hydrochemical characteristics and controls essential for elucidating regional water cycles and climate-hydrosphere interactions. While prior studies have examined broad hydrochemical patterns of Qinghai-Xizang Plateau lakes, systematic analyses distinguishing the unique signatures of different lake types—particularly within the Qinghai Province—remain scarce. This study presents a comprehensive assessment of 12 lakes (freshwater, saline, and salt lakes) in northeastern Qinghai-Xizang Plateau, integrating surface water and sediment porewater sampling. The results show the following: (1) In northeastern Qinghai-Xizang Plateau, for all three lake types, ion concentrations in sediment porewater are consistently higher than those in the overlying water, forming a near-bed concentration gradient. Salt lakes exhibit the highest absolute ion concentrations and the lowest spatial coefficient of variation. (2) All lakes are alkaline. Dominant cations and anions in freshwater and saline lakes are Ca²⁺, Na⁺, and SO₄²⁻, whereas salt lakes exhibit elevated proportions of Cl⁻, SO₄²⁻, Ca²⁺, and Mg²⁺. (3) Lake hydrochemistry is predominantly classified as either SO₄-Ca·Mg or Cl-Na types. Freshwater and saline lakes are primarily characterized as SO₄-Ca·Mg type, while salt lakes show notable differences between overlying water and porewater compositions. Lake water chemistry is primarily controlled by silicate weathering, carbonate dissolution, and evaporation-crystallization processes, with minimal influence from atmospheric precipitation across all lake types across the northeastern Qinghai-Xizang Plateau. This research provides crucial hydrochemical baselines for the understudied lake systems of the northeastern Qinghai-Xizang Plateau, offering vital insights into the ongoing hydrogeochemical responses to climate change on the Qinghai-Xizang Plateau.</p>