Trees are invaluable and multifunctional elements of many land cover types. In urban areas trees serve particularly important ecological and environmental functions. In order to manage the multi-functionality of trees as efficiently as possible, biophysical information on the presence and distribution of species is helpful, as is general information about their provenance, utilization and services. This study examined the tree species diversity along the rural-urban gradient in the rapidly expanding megacity of Bengaluru, India, where the focus was on density and species composition. A number of 23 observation plots of 1-ha each in built-up environments were established within a 50 × 5 km transect running from the city center to the rural surroundings. Tree species diversity decreased with increasing distance to the city center due to the abundance of non-native ornamental tree species in the urban areas. The percentage of native species was about the same along the gradient (around 35% in urban, transition and rural areas). Our study offers a comprehensive description of tree species diversity along the rural-urban gradient in Bengaluru, which may bear connections, for example, to bird and insect diversity, and which may also be of interest and relevant for city planners and administrators as well as for researchers and the general public. Moreover, our study does adds to the relatively limited information available on this topic in South Asia and suggests a design that is transferable to other urban areas. In addition, we hope to raise a critical awareness that contributes to protecting and developing the unique tree cover in Bengaluru.