The palm family is a major source of invasive woody plants because of their long history of introductions outside their native range in tropical and subtropical regions for their use as food and ornamentals. Areca triandra Roxb. Ex Buch-Ham, was introduced to Sri Lanka in 1867 through the Royal Botanic Gardens, Peradeniya has been grown as an ornamental for over a century. This study shows its naturalization in the southwest humid lowlands and is invasive. Population and reproductive characteristics were studied in Meethirigala and Yagirala Forest Reserves. In each location, stems > 1m height and crown cover were sampled in three plots of 400 m2 (20 m × 20 m) and the ground layer < 1 m height sampled through four 1 m x 1 m plots within each 400 m2 (total of 24 plots). A. triandra accounted for 56% - 66% of stems > 1 m height, 52% - 76% crown cover, and 64% - 77% of stems and seedlings in the ground layer < 1 m height. Clump formation was high with 30% - 51% having 2 - 7 stems, compared to single stems of 12% - 26%. Fruit production was prolific, each infructescence yielding 140 - 250 fruits and each clump of multiple stems producing 1 - 3 infructescence. This resulted in a total of 250 - 440 fruits per clump, translating up to 8000 fruits per 20 m × 20 m plot; bright red, fleshy fruits attracted avian and mammalian frugivores aiding seed dispersal. The palm favored disturbed forest edges, and shady understory habitats displacing native understory, suppressing native canopy regeneration. The Invasiveness Assessment Protocol indicated moderate risk from A. triandra in the wet zone, exposing vulnerability of Sri Lanka’s highly fragmented biodiverse rain forests. This emphasizes the need for urgent action of this species.