Seagrass species have a high source of bioactive compounds and are potential to be developed in health and medicinal application. One of the seagrass species, Thalassia hemprichii, was collected from Pramuka Island, Kepulauan Seribu National Park, Jakarta. Samples were cut into different parts, i.e., leaf, rhizome, and root. Each part of the samples was dried as a powdered simplicia and extracted using methanol (1:3; w/v). All the extracts were characterized using High Performance Liquid Chromatography (HPLC) and tested as larvicide against the larvae of Aedes aegypti. Batches of 25 early 3rd instar larvae of Ae. aegypti were transferred into 250 mL sample bottles containing 1% extract of 100 mL distilled water. There are two control groups: abate (0.01%) and distilled water of each has the same volume as the treatments. The mortality of individual larvae was observed after 12, 24, and 48 hours. The leaf and root extract showed 100 % mortality while the rhizome extract showed 56 % mortality. There is no mortality of 3rd instar larvae of Ae. aegypti in the control sample bottle containing distilled water, while abate as the positive control showed 100 % mortality. Chromatogram results from 3 different parts of T. hemprichii showed a similar pattern of peaks except for the leaf extract that has an additional peak at Retention Time (RT) 5.877. The additional peak probably plays a role to enhance the larvicide activity and toxic to Ae. aegypti larvae. The results suggested that extract of T. hemprichii have the potential to be used as a larvicide against Ae. aegypti larvae.