Sorghum halepense is a synanthropic tall grass distributed worldwide from tropical to temperate zones, and it is often considered an invasive alien. It is a perennial, rhizomatous plant that tends to form dense stands derived from vegetative and sexual propagation. Despite roadside plant communities dominated by Sorghum halepense are very common in southern Europe, their phytosociological aspects are scarcely studied. In this work, we present the results of a vegetation survey in peninsular Italy, carried out by means of the phytosociological method. In total, we carried out 73 relevés in Liguria, Tuscany, Latium, Campania, Basilicata, and Apulia. We statistically compared our relevés to those from the Balkans classified in the Cynodonto-Sorghetum halepensis , an association of agricultural annual weed vegetation of the class Stellarietea mediae s.l. used in the past as a reference for Italian S. halepense -dominated communities. Our results show that the Italian communities are different from the Cynodonto–Sorghetum halepensis communities, since the latter are rich in annual species, while the former are rich in perennial species. From the syntaxonomic point of view, the Italian communities are better classified in the class Artemisietea vulgaris . We describe the new (sub-)ruderal association Potentillo reptantis–Sorghetum halepensis , including a meso-hygrophilous variant with Urtica dioica and an agricultural variant with Elymus repens . We have evidence that the Potentillo–Sorghetum occurs in Italy, Kosovo and Slovenia, but its distribution is possibly wider due to conspicuous presence of cosmopolitan species characterizing the association. Our work provides a baseline for the knowledge of an alien-dominated plant community that can invade habitats with high conservation value.