This study explores the diversity of flowering trees within the Nawala premises of the Open University of Sri Lanka (OUSL). Situated in a highly urbanized area with a landfill marsh, the OUSL spans 133,216.3 m² and includes a variety of landscapes such as internal roads, buildings, lawns, gardens, and forested regions. Employing a multifaceted research approach, the study integrates both primary and secondary data collection methods. Primary data was obtained through extensive surveys conducted across various zones of university, focusing on the identification and classification of plant species using taxonomic keys. Additionally, consultations with university experts provided valuable historical insights into changes in plant diversity and their environmental implications over time. The study identified a total of 1,247 plants across six zones, revealing 156 species from 75 genera and 44 families. This included 6 endemics, 111 indigenous, and 39 exotic species. The Fabaceae family was the most represented, with 21 species, followed by Myrtaceae with 15 and Rubiaceae with 9. Notably, Mangifera indica was the most abundant species, with 92 individuals recorded. Data analysis using the Shannon diversity index yielded values of 4.40 for OUSL and 4.03 for zone D. These results demonstrate that OUSL supports significant plant diversity despite its urban context, positively contributing to environmental sustainability. The study advocates for enhanced conservation efforts and the promotion of native species to further bolster environmental friendliness, underscoring the role of educational institutions in advancing urban biodiversity conservation and sustainable practices. published by the Journal of Biodiversity and Environmental Sciences (JBES)