The introduction of Acacia saligna in the north Nile Delta of Egypt is causing harmful impacts on the plant diversity. Thirty stands were established in both invaded and non-invaded areas. The species abundance and diversity were determined. Soil samples were collected and analyzed. The correlation between vegetation and soil variables was investigated. The allelopathic potential of water and methanolic extracts from A. saligna leaves and flowers were examined. Approximately 19 and 50 plant species were recorded in the invaded and non-invaded areas, respectively. Aegilopus bicornis was the dominant species in the invaded areas, while Senecio glaucus dominated the non-invaded areas. The non-invaded areas attained high values of species richness and lower values of evenness than invaded one. Soil analysis revealed that the non-invaded areas attained significantly high content of sulphate, bicarbonate, Na, K and Ca than invaded areas. The methanolic and aqueous extracts of the flowers attained IC50 values of 2.89 g/L and 33.89 g/L, respectively on the germination of Hordeum murinum. However, IC50 values for leaves were 6.08 g/L and 55.04 g/L, respectively. The methanolic extracts of A. saligna expressed more effect on seedling length than the aqueous extracts. The aqueous extract showed stimulatory effect at low concentrations. The invasive successes of A. saligna seem to be related to its ability to release allelopathic compounds together with its competition for resources such as nutrient, water and sunlight. These findings may also have useful implications for coastal ecosystem management and conservation in Egypt