Long-term climate variability is increasingly recognized as a driver of societal change, yet its contribution to early state formation on the Tibetan Plateau remains poorly understood. Here we integrate high-resolution paleoclimate proxies, historical documentation, and an 8000-year transient simulation with the EC-Earth3 Earth system model to examine how multicentennial variability in the Atlantic Meridional Overturning Circulation (AMOC) influenced the rise and fall of the Tubo Empire (633–877 CE). We show that a strong AMOC induces a northward-shifted Intertropical Convergence Zone (ITCZ), an intensified Indian summer monsoon, and a ~5–10% increase in summer precipitation across the southwestern Plateau—conditions consistent with the empire’s expansion and demographic growth. A weakened AMOC produces the opposite hydroclimatic regime, aligning with the empire’s late ninth-century fragmentation. Spatial and dynamical analyses reveal that AMOC-driven North Atlantic sea-surface temperature anomalies modulate Tibetan hydroclimate primarily through the monsoon–ITCZ pathway rather than midlatitude wave trains. These results demonstrate that internally generated ocean–atmosphere variability created a persistent environmental backdrop for societal transformation on the Plateau. Recognizing such multicentennial modes is crucial for reconstructing human–climate interactions in the past and assessing future water-security risks in the context of a potentially weakening modern AMOC.