Biological invasions are the second biggest source of ecosystem destruction. But knowledge of invasive taxa is still lacking in Burundi. To support invasive plant managers, a study was carried out in the marshes of the Ruvubu River, Burundi's largest river. The aim was to: Carry out a floristic inventory; highlight the diversity of invasive plants and determine a floristic gradient. The phytosociological method was used to establish floristic richness. The determination of invasive plants required a combination of the average cover rate and the biological characteristics of the invasive specific plasticity of the dominant species. The results showed 161 species in 125 genera and 54 families. 13 proven invasive species and seven other potential invasive plant species were retained, spread across six botanical families, two of which (Poaceae and Asteraceae) are the most represented. The floristic gradient of invasive plants increases from upstream to downstream.