The chestnut-flanked sparrowhawk has blackish-grey upperparts with a very distinctive pattern on the underparts; the breast and belly are heavily barred grey and brown, with chestnut-colored flanks. The throat is white, and the head is rather broad compared to similar species. The cere is yellow, as is the thin eyering which surrounds the red eye. Females and juveniles are browner. They stand tall and have a wingspan of.
The habits of the chestnut-flanked sparrowhawk are poorly known but it is known to layeggs during January to April in Gabon. It probably feeds mainly on birds but has been recorded catching bats. As it is rarely seen in the open it is presumed to be a still hunter which sits in the cover of foliage and sallies out to catch prey. Has been known to enter houses after poultry and to follow driver ant columns to ambush the attendant small birds.