Dormer's television credits include "Game of Thrones" and "The Tudors."
"We're moving to the next chapter in our show -- and in some ways, to a previous chapter in Holmes' story," executive producer Rob Doherty said in a statement Tuesday. "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."
Dormer's arc is to begin May 9 and will end with the show's 2-hour Season 1 finale.
Based on the 19th century stories by Sir Arthur Conan Doyle, "Elementary" is a modern-day drama about private detective and recovering drug addict Sherlock Holmes and former surgeon Joan Watson, a crime-solving duo called upon to help crack the New York Police Department's toughest cases.