Coat color is exclusively black and their hair is longer but not as dense as other gibbon species. A throat sac is present which expands during vocalizations. Calls have been heard over 4 km with females producing the longest sounds. The call itself is deep and resonating. The main song is a duet between the pair with additional notes provided by the offspring.
Siamangs sing less frequently than the other gibbons.
Typically found 25 or more meters high. Approximately half the diet is leaves. Roughly forty percent is fruit. Other plant parts, insects, and small vertebrates make up the small remainder. Groups are made up of bonded pairs and up to three of their offspring.