Floristic research of alien flora in lower course of the Sava River basin was conducted on the localities Sremska Mitrovica, Jarak (municipality of Sremska Mitrovica), Šabac, 'Crni Lug' (Boljevci, Surčin) and area between Ostružnica and Makiš (including Makiš). Research in- cluded water body, flood zones, mounds, canals, abandoned and arable land close to the riv- erbed or flood zone. It was detected 48 alien plants, grouped into 24 families, among which Asteraceae (29.2%), Poaceae (10.4%) and Fabaceae (8.3%) stands out by number of alien species. The biological spectrum is therophyte type, chronological spectrum is dominated by neophytes and chorological spectrum is dominated by species of North American origin. The most common dispersal methods are dispersal by humans, animals and wind. In the Sava River riverbed three alien macrophytes were recorded (Elodea canadensis, Vallisneria spiralis and Paspalum paspaloides), while in other areas the most frequent species were Acer negundo, Morus alba, Fraxinus lanceolata, Amorpha fruticosa, Ambrosia artemisiifolia, Stenactis anua, Amaranthus retroflexus, and species of genera Xanthium and Solidago. In order to prevent degradation of the natural and anthropogenically altered ecosystems in the lower course of the Sava river, caused by the introduction of alien species, their naturalisation and dissemination, it should take appropriate preventive measures. The most effective strategy in fighting against invasive species is to prevent their invasion.