Abstract: Southwest China (SWC) is one of the major grain-producing areas in China, and the surface air temperature (SAT) during autumn has a substantial influence on grain production and planting. It is therefore important to understand temporal changes in the SAT over SWC (SWC-SAT). Our analysis of observational and reanalysis datasets shows that the autumn SWC-SAT exhibits significant multidecadal variability. A significantly strong positive correlation also exists between the autumn SWC-SAT and the Atlantic multidecadal oscillation (AMO) time series (correlation coefficient of 0.85). These results suggest that the AMO is a remote driver of multidecadal variability in the autumn SWC-SAT. Further analyses show that the North Atlantic sea surface temperature anomalies (SSTA) associated with the AMO modulate the multidecadal variability of the autumn SWC-SAT through triggering the Africa–Asia multidecadal teleconnection (AAMT) pattern. Specifically, the AAMT corresponds to geopotential height anomalies over SWC, which adjust the local thickness of the air column and thereby induce multidecadal variability of the autumn SWC-SAT. This potential mechanism, derived from observational and reanalysis datasets, was verified by using a linear barotropic model and the Community Atmosphere Model version 4. Our results from combining observations and numerical modeling simulations indicate that the North Atlantic SSTA may act as a key pacemaker for the multidecadal SAT variability over SWC.