Research on the summertime runoff variations and their connections with Asian summer monsoons can give insights for explanation of the hydrological processes and climate change in the Yangtze River basin. Currently, regional studies are focused on the relationships between Asian summer monsoons and meteorological elements. However, research on the runoff variations and their connections to Asian summer monsoons is still scarce. With the help of continuous wavelet transform, cross-wavelet, and wavelet coherence analysis methods, this research explored multiscale summertime runoff variations and their connections with Asian summer monsoons during 1957–2012 in the Yangtze River basin. The results indicate that periodical characteristics of summertime runoff along the mainstream of the Yangtze River basin have distinct differences. Upstream flow is characterized by interannual (1- to 3-year), and downstream by decadal (7- to 10-year) oscillations over certain time periods. In the source region, summertime runoff is primarily influenced by the South Asian summer monsoons (SASM), and mainly in-phase relationships are detected between the summertime runoff and SASM indices. In the midstream and downstream regions, summertime runoff is primarily influenced by the East Asian summer monsoons (EASM), and mainly anti-phase relationships are detected between the summertime runoff and EASM indices.