Anthropogenic pressure, increasing climate variability and climate change are causing severe degradation of ecosystems. Therefore, in order to be able to guide their conservation, it is necessary to have as much information as possible on the state of their biodiversity. The overall objective of this thesis is to contribute to the characterization of the biodiversity of Ruvubu National Park (PNR), the largest protected area in Burundi, through the analysis of its floristic composition, the structure of its plant communities and ecological parameters. The work combined diverse approaches, including vegetation and soil sampling and laboratory analyses. The study thus provided the first complete list of vascular flora in the PNR including 515 species divided into 98 families and 309 genera. Ninety-six species not yet recorded in the protected area have been discovered. The most abundant families are Fabaceae, Asteraceae, and Poaceae. Savannas, forest galleries, and marshes are dominated by hemicryptophytes, phanerophytes, and geophytes respectively. Ten plant groups, five of which are new, present in three classes, four orders and seven phytosociological alliances have been individualized. The groups generally have relatively low taxonomic diversity while equitability suggests disturbed communities, particularly in savannas. Altitude and four physicochemical parameters of the soil (total nitrogen, total carbon, stony load and pH) explain 48% of floristic variability; the latter being significantly influenced by altitude. However, floristic variability unexplained by environmental factors suggests the involvement of other factors. In terms of conservation, thirty species are indigenous woody plants, priority for revaluation and multiplication in Burundi. Ten plant species are also on the list of endangered species that are priority for conservation in Burundi. In short, the fact that the PNR houses several elements of biodiversity protected by the international community gives it an important function as a reservoir of biodiversity. All these assets speak in favor of strengthening strategies for the conservation and rehabilitation of its biodiversity in all its components.