Fatumasin is a mountain in the district of Liquiçá in East Timor, a country occupying the eastern end of the island of Timor in the Lesser Sunda Islands of Wallacea.

The surrounding forest is called Hutan Gunung Maelulu in Indonesian.

The mountain stands about 26 km south-west of the national capital of Dili.

The underlying rocks are non-calcareous and the forest is considered to be more species-rich than most others on the island.

[1] The site has been identified by BirdLife International as an Important Bird Area (IBA) because it supports populations of bar-necked cuckoo-doves, pink-headed imperial pigeons, yellow-crested cockatoos, jonquil parrots, streak-breasted honeyeaters, Timor friarbirds, black-breasted myzomelas, plain gerygones, fawn-breasted whistlers, green figbirds, olive-brown orioles, Timor stubtails, Timor leaf warblers, orange-sided thrushes, Timor blue flycatchers, blue-cheeked flowerpeckers and flame-breasted sunbirds.