The past agrobiodiversity and evolution of barley, a staple cereal in the northwestern Mediterranean region for the last ca 8000 years, is still poorly documented. This study employed an intensive sampling strategy to analyse morphometric variation of 9817 grains (264 samples and 102 sites), dating from the Neolithic to the Late Middle Ages, from southern France and Catalonia. We utilized an archaeophenomic approach to quantify grain size and shape through elliptic Fourier transformation. We contrasted the variation of archaeological grains with that of 6397 caryopses from 105 modern Euro-Mediterranean varieties of 2-row/6-row subspecies, hulled/naked types. Past climate conditions in southern France/Catalonia were investigated to provide an environmental framework for interpreting morphometric variation. Barley grains increased in size, and changed in shape over time, likely owing to a combination of factors, including environmental adaptation and human selection, coupled with changes in agricultural practices. Naked barley was prevalent during the Neolithic, while the proportion of hulled barley increased from the Late Bronze Age onwards. In contrast, subspecies were identified in all assemblages, without clear temporal trends but with strong variation between assemblages. This study uncovers a complex interplay of environmental and human factors in cereal evolution, highlighting the evolution of agrobiodiversity that fuelled the development of modern societies.This article is part of the theme issue 'Unravelling domestication: multi-disciplinary perspectives on human and non-human relationships in the past, present and future'.