The Eastern Arc and Coastal forests of Tanzania are biodiversity hotspots, centres of endemism, and highest priority ecosystems for biodiversity conservation worldwide. But, their floristic similarity and species diversity gradients over geographical distances and longitudinal locations, respectively, are poorly understood. Thus, this study investigates the relationship between floristic similarity and geographical distances, as well as species diversity and longitudinal locations among the forests. A total of 37 inventories from the Eastern Arc and Coastal forests of Tanzania were selected based on inventories with minimum diameter at breast height (DBH) ≥ 10cm), and identification of trees done at species level. The floristic similarity was determined using the Sørensen’s similarity coefficient. The Pearson correlation test was used to study differences between floristic similarities and geographical distances, as well as species diversity and longitudinal location. The findings revealed three clusters of forests assembled due to their geographical proximity. The correlation between geographical distances and floristic similarities revealed a significant negative association (n = 528, R2 = 0.1821, p < 0.001), while species diversity and longitudinal location of forests had a significant positive correlation (n = 37, R2 = 0.5401, P < 0.05). The results suggest that high environmental heterogeneity across the forests should be well thought-out in their management plans to uphold their valuesDar es Salaam University College of Education (DUCE) and the Deutscher Akademischer Austausch Dienst (DAAD).