The actress's spokesperson confirmed she had accepted the Klaxons rocker's proposal following a 15 month courtship.
The 27-year-old started dating Righton last year after being introduced by mutual friend Alexa Chung, the Daily Mail reported.
"I can confirm that Keira is engaged to be married to James Righton," her representative said.
The Oscar-nominated star previously dated Rupert Friend for five years before splitting in January 2011. She also dated Irish model and actor Jamie Dornan before splitting in August 2005.