The "Pirates of the Caribbean" star, 27, accepted the Klaxons rocker's proposal following a 15 month courtship, reported Daily Mail.
The London-born actress started dating the 28-year-old keyboard player last year after being introduced by mutual friend Alexa Chung.
"I can confirm that Keira is engaged to be married to James Righton," Knightley's representative said.
In keeping with the actress' guarded privacy stance, the spokesperson declined to comment further.
Since they started dating last year, the couple have kept their romance under wraps and are yet to pose together on a red carpet.
However, they have been photographed out together casually near their East London home several times.
The Oscar-nominated actress previously dated "Pride & Prejudice" co-star Rupert Friend for five years before splitting in January 2011.