Simple SummaryBiological invasions, i.e., colonization and the spread of species outside their native range, are among the main threats to biodiversity conservation. The Central and South American macrophyteLudwigia hexapetala, which is considered invasive in Italy, can grow both in slow-flowing waters and along the banks of lakes, rivers, and canals. In this study, field data were collected to assess the impact of this alien species on the environmental parameters and plant diversity of the invaded habitats. The results show that aquaticL. hexapetalapopulations, characterized by leaves floating on the water surface, alter the habitats they colonise by reducing the available light and the amount of dissolved oxygen in the water. Moreover, these populations negatively affect aquatic plants as increasing levels of invasion correspond to a decrease in plant diversity. Conversely, bankL. hexapetalapopulations do not have the same negative impact on bank plant diversity. In fact, some bank plants, such as the nativePhragmites australis(common reed), may form a barrier toL. hexapetalainvasion by producing extensive and dense populations. Based on this evidence, promoting the conservation of key aquatic and bank native plants that can counterL. hexapetalainvasion could be a useful management practice in environments at risk of invasion by this alien species.AbstractBiological invasions are a serious threat to biodiversity conservation, especially in freshwater ecosystems. The American macrophyteLudwigia hexapetala, which colonizes both the aquatic and bank habitats of lakes, rivers, and canals, is invading many waterbodies in Europe, becoming an increasingly worrisome threat in several European countries, including Italy. However, only fragmentary information is available on the actual impact of its invasion in these habitats. This study aims to collect field data from various freshwater habitats in central and northern Italy, to assess the possible impact ofL. hexapetalaon the environmental parameters and plant biodiversity of the invaded habitats. The results show that in aquatic habitats, dense floatingL. hexapetalapopulations reduce the light levels and oxygen available in the water, consequently limiting the growth of other aquatic plants. Indeed,L. hexapetalapopulations negatively affect aquatic plant diversity, as an increase inL. hexapetalacover corresponded to a decrease in Simpson’s diversity index. In contrast, in bank habitats,L. hexapetalahas no significant impact on plant diversity. Evidence suggests that some native species, such asPhragmites australis, which generally form compact populations along the banks, effectively counteract the invasion ofL. hexapetala. This information may prove valuable for the environmental managers of those freshwater habitats whereL. hexapetalainvasion needs to be addressed and controlled.