The siamang (Symphalangus syndactylus) is an arboreal black-furred gibbon native to the forests of Malaysia, Thailand, and Sumatra. The largest of the gibbons, the siamang can be twice the size of other gibbons, reaching 1 m in height, and weighing up to 14 kg. The siamang is the only species in the genus Symphalangus.

The siamang is distinctive for two reasons. The first is that two digits on each foot are partially joined by a membrane—hence the name "syndactylus", from the Ancient Greek sun-, "united" + daktulos, "finger". The second is the large gular sac (found in both males and females of the species), which is a throat pouch that can be inflated to the size of the siamang's head, allowing the animal to make loud, resonating calls or songs.

There may be two subspecies of the siamang. If so, they are the nominate Sumatran siamang (S. s. syndactylus) and the Malaysian siamang (S. s. continentis, in peninsular Malaysia).Otherwise, the Malaysian individuals are only a population. The siamang occurs sympatrically with other gibbons; its two ranges are entirely within the combined ranges of the agile gibbon and the lar gibbon.

The siamang can live to around 40 years in captivit