This paper investigates the eastern movements of barley grains and their morphological variations in prehistory. By combining previously published and newly collected archaeobotanical grain measurements (n = 2,176), we explore the roles of culinary traditions underlying the morphological traits observed. We find that barley diminished in size as it moved from its origin in southwestern Asia to Central and East Asia between the third millennium BC and first millennium BC. In particular, the grains in Monsoonal China became greatly reduced in comparison to other regions as the crop was incorporated into eastern small grain cuisines. The reverse pattern is observed in the high-altitude Tibetan environment, which is attributed to the practicalities of cooking under low vapour pressure conditions. These results, demonstrating that barley moved eastward but western grinding and baking traditions did not, reveal the complexity of the eastern culinary system and raise awareness of decoupling of grains and their associated cuisines.