Mount Katayagan, the tallest peak in the Agoo Mountain Range of the Province of La Union, Philippines, is a crucial yet understudied natural resource. This research addresses the biodiversity gap in non-protected areas like Mt. Katayagan, emphasizing its role as a watershed, supplying water for irrigation and households, and providing essential commodities such as fruits and timbers. Despite its ecological and economic significance, there is a lack of research on Mt. Katayagan’s biodiversity. This study aims to fill this gap by conducting a comprehensive flora assessment, identifying land cover types, forest formation, stand maturity and dominant upper canopy species. Using the “Terrestrial Ecosystem Biodiversity and Assessment Monitoring Manual,” the research reveals Mt. Katayagan’s predominantly closed forest with characteristics of a secondary-growth tropical moist deciduous forest. Pterocarpus indicus (narra) emerges as the dominant upper canopy species with an Importance Value of 20.85 percent. This research provides essential baseline data for environmental protection laws and emphasizes the urgent need for collaborative conservation efforts involving local communities, government bodies, and research institutions to safeguard Mt. Katayagan’s unique biodiversity. published by the Journal of Biodiversity and Environmental Sciences (JBES)