Like other toucans, the Chestnut-mandibled is brightly marked and has a large bill. The male is 56cm long and weighs 750g (26.5 oz). The smaller female is typically 52cm long and weighs 580g (20.5 oz).
The sexes are alike in appearance, mainly black with maroon hints to the head, upper back and lower breast. The face and upper breast are bright yellow, with narrow white and broader red lines forming a lower border. The upper tail is white and the lower abdomen is red. The legs are blue. The body plumage is similar to that of the smaller Keel-billed Toucan, but the bill pattern is quite different, being diagonally divided into bright yellow and maroon.
Juvenile birds are sooty-black, and have duller plumage, particularly with respect to the bib, red border, and lower mandible. They are fed by the parents for several weeks after leaving the nest.
The call of the Chestnut-mandibled Toucan is a yelping yo-YIP, a-yip, a-yip, or a Dios te dé, Dios te dé. It is given to maintain contact as the flock travels in "follow-my-leader" style through the trees, but also in chorus at the evening roosts.