A critical gap in East Asian summer monsoon research is the lack of speleothem records from its northeasternmost fringe, hindering tests of orbital-scale monsoon variability at higher latitudes. Our new records from the Russian Far East, spanning 9-5 ka BP, fill this spatial gap and reveal a synchronous delta O-18 negative excursion and intensified monsoon precipitation between 8 and 9 ka BP, contrasting with trends in North China. We suggest this difference may stem from changes in Northern Hemisphere summer insolation (NHSI) over the Holocene. The NHSI decrease since early Holocene weakens southwesterly winds and the Western Pacific Subtropical High, shifting the westerly jet and rainbelt southward. This causes reduced (increased) precipitation in North China (Northeast Asia), as well as reduced (increased) contribution of Pacific Ocean moisture to North China (Northeast Asia), resulting in delta O-18 increase (decrease) in North China (Northeast Asia). Plain Language Summary Speleothem oxygen isotope (delta O-18) records are widely used as high-resolution proxies to characterize East Asian summer monsoon (EASM) variability at various timescales. While discrepancies between coherent delta O-18 records and regionally inconsistent moisture patterns have been identified, studies on the eastern edge of the EASM region remain sparse. This study provides new cave records from the Russian Far East, covering 9 to 5 ka BP, the northernmost site in the EASM region. Combined with other records from Northeast Asia, we observe significant synchronous depletion of speleothem delta O-18 and intensified monsoon precipitation in Northeast Asia between 8 and 9 ka BP. This contrasts with patterns in North China. We suggest that this contrast is driven by changes in the Northern Hemisphere Summer Insolation (NHSI) during the Holocene. A decrease in NHSI weakens the southwesterly winds and the West Pacific Subtropical High, while simultaneously inducing a southward shift of the westerly jet. These changes collectively force the East Asian rain belt southward. This leads to reduced precipitation in North China and increased rainfall in Northeast Asia, along with delta O-18 enrichment in North China and depletion in Northeast Asia. These findings offer a more spatially detailed understanding of the EASM region's footprint from North China to Northeast Asia during the Holocene.