Hylobates syndactylus is the largest of the gibbons, weighing between 10 and 12 kg. The head-body length ranges from 71 to 90 cm. They have a thick, black fur coat and long, slender arms. The arm length may attain 2.3 to 2.6 times the body length. Both sexes have long canine teeth, opposable thumbs, and a great toe that is deeply separated from the foot. Hylobates syndactylus has a short-muzzled face that is nearly hairless, accompanied by a large brain case.

The most distinguishing characteristic of siamangs is the enlarged throat sac that can be as big as a human head! These throat sacs are used as a sound box to amplify their loud vocalizations.

Siamangs are syndactylous, having their 2nd and 3rd toes fused by a thin webbing of skin.

(Preuschoft, 1990; Chivers, 1979) (Nowak, 1999)