Recent works have revealed that the wintertime atmospheric river (AR) activity is closely related to the 30–60-day tropical intraseasonal variability, yet it remains unclear whether summertime AR activity is also significantly influenced by intraseasonal variability, often referred to as the boreal summer intraseasonal oscillation (BSISO). Diagnosing the 40-yr (1979–2018) ERA5 reanalysis dataset, the present study examines the climatological features of ARs over the Indo-Pacific region during June–October and its associations with the BSISO. Results suggest that the western North Pacific subtropical high (WNPSH) provides a favorable circulation background for the summertime AR activity, which conveys the moisture from the tropics to the midlatitude North Pacific along its periphery. Our analysis reveals that the BSISO has substantial impacts on the occurrence and distribution of ARs. More ARs are found over the western North Pacific (WNP) when the BSISO convective envelope propagates northward to the subtropical regions, while fewer ARs can be seen when convection is suppressed there. Specifically, in phases 7–8, the active BSISO convection over the Philippine Sea induces a low pressure anomaly and the corresponding anomalous cyclonic circulation, leading to the enhanced poleward moisture transport and more frequent AR activity over the WNP. Moreover, the WNP ARs tend to be longer and have larger sizes during these two phases. It is also found that more frequent occurrence of tropical cyclones in phases 7–8 can significantly enhance the moisture transport and AR occurrence over the WNP.