Mangroves contain significant biodiversity and are highly valuable for coastal communities for their daily life and ecotourism attractions. It was observed that appropriate environmental and soil factors are important for conducive growth of mangroves, which in turn influence species distribution, composition and diversity. This study was carried out to compare soil fertility status among three main riverine systems and to analyze the effects of environmental and soil fertility factors on species richness, evenness and diversity in mangrove forest at Kilim Geoforest Park in Langkawi. A total of one hundred (20 m × 20 m) plots were established along both sides of the three rivers: River Kisap (40 plots), River Ayer Hangat (30 plots) and River Kilim (30 plots). All the species and trees of diameter 1 cm dbh and above found within the plots were enumerated and identified. Species richness was computed based on the Jackknife method and species diversity using Simpson`s Index, Shannon-Wiener`s Index and Brillouin`s Index. The evenness index was measured by Simpson`s measure of evenness, Camargo`s index of evenness and Smith and Wilson`s index of evenness. The composite soil samples of each riverine system were analysed for available P, dry Ph, total N & C and exchangeable (Ca, K, Mg & Na). The Canonical Component Analysis (CCA) method was used to show the relationship between the environmental and soil fertility factors and plant species diversity. Based on the analysis of the current study, soil fertility factors in the study areas were significantly different among the three rivers. Meanwhile, biodiversity indices like Brillouin`s index, Shannon-Wiener index, Simpson`s index, and Jackknife estimates of species richness were clustered together in River Kisap, as also indicated by the clustering of mangrove species such as Avicennia marina, Acanthus volubilis, Bruguiera cylindrica, Ceriops decandra, Xylocarpus moluccensis, Rhizophora apiculata and Bruguiera parviflora. Meanwhile, the ordination diagram of canonical redundancy analysis showed strong correlations between environmental and soil fertility factors and species diversity in Kilim Geoforest Park, Langkawi.