Climatic changes during the first half of the Common Era have been suggested to play a role in societal reorganizations in Europe (1,2) and Asia (3,4). In particular, the sixth century coincides with rising and falling civilizations (1–6), pandemics (7,8), human migration and political turmoil (8–13). Our understanding of the magnitude and spatial extent aswell as the possible causes and concurrences of climate change during this period is, however, still limited. Here we use tree-ring chronologies from the Russian Altai and European Alps to reconstruct summer temperatures over the past two millennia. We find an unprecedented, long-lasting and spatially synchronized cooling following a cluster of large volcanic eruptions in 536, 540 and 547 AD (ref. 14), which was probably sustained by ocean and sea-ice feedbacks (15,16), as well as a solar minimum (17). We thus identify the interval from 536 to about 660 AD as the Late Antique Little Ice Age. Spanning most of the Northern Hemisphere, we suggest that this cold phase be considered as an additional environmental factor contributing to the establishment of the Justinian plague (7,8), transformation of the eastern Roman Empire and collapse of the Sasanian Empire (1,2,5), movements out of the Asian steppe and Arabian Peninsula (8,11,12), spread of Slavic-speaking peoples (9,10) and political upheavals in China (13).