Abstract: The Sanjiangyuan region is an important ecological security barrier in China． Understanding interactions among ecosystem services (ESs) and their driving factors across different river basins is of great significance for the management of watershed ecosystems． In this study, InVEST model, correlation analysis, and K-means clustering were used to evaluate spatiotemporal patterns of ecosystem services, trade-offs and their synergy relationships in 2010 and 2023． Redundancy analysis (RDA) was conducted to identify associations between ecosystem services and environmental as well as socioeconomic factors． Significant differences in ecosystem service provision among river basins were found in the Sanjiangyuan region． The Yellow River Source basin and the Lancang River Source basin exhibited relatively high levels of ecosystem service supply, whereas the Yangtze River Source basin showed comparatively low levels． Synergistic relationships predominated among ecosystem services． A trade-off relationship between carbon sequestration and water conservation was observed in the Lancang River Source basin, whereas a synergistic relationship was identified in both the Yangtze River Source basin and the Yellow River Source basin． Vegetation coverage, precipitation, and slope were the factors most strongly associated with ecosystem services． In the study period, the habitat–carbon sequestration function was enhanced in the Yellow River Source basin, but agro-pastoral recreation function was predominant in the Yangtze River Source basin． Under the combined effects of climate change and ecological restoration projects, ecosystem services in the Sanjiangyuan region generally exhibited an increasing trend． In the future, ecological conservation in the Yangtze River Source basin could be further strengthened through measures such as artificial grassland establishment and sustainable grazing management． These findings will provide some scientific basis for differentiated ecological management and conservation strategies in the Sanjiangyuan region．