The actress will play Queen Margaret in the three BBC films, a spin-off of William Shakespeare's Henry VI trilogy and Richard III. Okonedo's character is the only one who appears in Henry VI Part 1 and is still alive at the end of Richard III.
Okonedo, who is currently making her Broadway debut with Raisin, will play the queen who curses Cumberbatch's Richard III.
The Henry VI plays, which will be divided into two films, and Richard III will begin filming in late September.
Dominic Cooke will direct all three of the new "Hollow Crown" dramas.
Click here to read the Playbill.com feature about Okonedo and her "euphoric, incredible" Broadway debut.