The Siamang is the largest of the lesser apes and has extremely long arms, that are longer than its legs. Remarkably, it has two fingers on each hand that are fused together, hence the name syndactylus.

It has a black fur, with long, dense, shaggy hair, which on the head has a neat center parting, and an inflatable gular pouch, which inflates when the animal is calling, producing a call that may carry for several kilometers when conditions are right.