The year 2019 marked the 40th anniversary of the outbreak of the Sino-Vietnamese War in 1979. Making use of published and unpublished Chinese, Vietnamese, and English sources, this article traces the tensions between official and popular memories of the Sino-Vietnamese War of 1979 in China and Vietnam, respectively. We argue that these tensions existed because the development of the official Chinese and Vietnamese memories of the war largely mirrored each other. Between 1991 and roughly 2006, when bilateral relations between the countries improved, both Beijing and Hanoi claimed victory for their side while simultaneously downplaying the bloodshed, tragedy, and loss experienced during the war. However, they have reacted to the rise of popular memories since the early 2000s in very different ways. While Beijing walks a thin line between accommodating appeals for greater recognition of the sacrifices made by ordinary soldiers without provoking social discontent with the political system, Hanoi has been more successful in mobilising Vietnamese popular memory of the war to strike a measured nationalistic response to China. How China and Vietnam remember and downplay the Sino-Vietnamese War points to the bigger picture of the sensitivity of bilateral relations to historical memory in Asia. In particular, historical memory shapes how a country perceives external threats and opportunities, while historical memory is created, suppressed, and re-created as international relations evolve.