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To investigate the effects of river waters on estuarine seagrass, the tropical seagrass Halophila ovalis was collected at Brunei Bay, Borneo, and was cultured under laboratory conditions for 18 days. Three treatments were set up in the experiment: natural seawater with a salinity 30 (S30), estuarine river water with a salinity 10 (S10), and the intermediate water that was composed of the seawater and river water with a salinity 20 (S20). New leaf production, the average length of new leaves, rhizome elongation, and photosynthetic rate of H. ovalis were significantly higher in S20 than S10. Chlorophyll a (chl a) and carotenoid content in H. ovalis were significantly lower in S10 than S20 and S30. Though the tropical river waters could potentially cause both positive and negative effects on seagrass, the present results suggested that low salinity would be the most influential factor to hinder the growth and metabolism of H. ovalis, and the salinity threshold was observed between 10 and 20. These results suggested that H. ovalis may be able to extend its present distribution to the upper estuary at this study site in the future. [ABSTRACT FROM AUTHOR]