Recent studies of the Alyssum montanum – A. repens complex in Europe showed that populations from central and southern Italy are, in contrast to previous taxonomic treatments, clearly distinct from A. montanum and should be classified as A. diffusum. Based on the high, geographically structured genetic variation (central Apennines, Gargano, and southern Apennines/Calabria) of A. diffusum, three subspecies were recently recognised. Here, we present a detailed morphometric study of A. diffusum, exploring its differentiation from A. montanum and differentiation among the three subspecies (A. diffusum subsp. diffusum, subsp. garganicum, and subsp. calabricum). We report that A. diffusum differs from A. montanum, particularly by having (on average) larger flower parts (from subsp. gmelinii) and different density and morphology of trichomes and width of petals (from subsp. montanum). The subspecies of A. diffusum can be distinguished by the number and length of trichome rays, by trichome density on the lower leaf surface, and by the leaf length. In this study, we present an identification key which includes the subspecies of A. diffusum, those of A. montanum and a related A. cuneifolium. We also provide detailed morphological descriptions of all three subspecies of A. diffusum and a nomenclatural account, including the designation of the lectotype of the name A. diffusum.