She will be portraying Sherlock's first love in London, Irene Adler. Her death caused the detective to spiral into substance abuse.
Executive Producer and creator Robert Doherty has this to say about the addition to the cast.
"We're moving to the next chapter in our show - and in some ways, to a previous chapter in Holmes' story. Irene Adler is perhaps the most pivotal relationship in Sherlock's life, and I can't think of an actress better suited to the challenge than Natalie."
Natalie Dormer will also be seen in Ridley Scott's upcoming thriller The Counselor and Ron Howard's action biography Rush.