The red-thighed sparrowhawk was formally described in 1885 by the German ornithologist Gustav Hartlaub based on a specimen collected in "Rio Bontry", now Butre on the coast of Ghana.

In 2024 a comprehensive molecular phylogenetic study of the Accipitridae confirmed earlier work that had shown that the genus was polyphyletic.

The genus Tachyspiza was resurrected to accommodate the red-thighed sparrowhawk together with 26 other species that had previously been placed in Accipiter.

The red-thighed sparrowhawk is sexually dimorphic, the males of the nominate subspecies having very dark grey upperparts with a white crescent on the lower rump which is conspicuous in flight, as are three white broken tail bars on the dark grey tail.

[12] The red-thighed sparrowhawk is a secretive and crepuscular species that spends most of the day perched in the interior of the forest.