Hoolock gibbons are two primate species of genus Hoolock in the gibbon family, Hylobatidae.

Hoolocks are the second-largest of the gibbons, after the siamang. They reach a size of 60 to 90 cm and weigh 6 to 9 kg. The sexes are about the same size, but they differ considerably in coloration: males are black-colored with remarkable white brows, while females have a grey-brown fur, which is darker at the chest and neck. White rings around their eyes and mouths give their faces a mask-like appearance.