The two are not releasing any details surrounding wedding plans or the proposal, said publicist Sara Keene.
Righton, who plays keyboard for the rock group Klaxons, and Knightley, 27, have been dating since early 2011. Previously, Knightley dated actor Rupert Friend for several years.
The actress rose to fame for her role as a soccer-playing teen in "Bend it Like Beckham." She went on to star in the first three "Pirates of the Caribbean" films and was nominated for an Oscar in 2006 for her role as Elizabeth Bennet in an adaptation of Jane Austen's "Pride and Prejudice."
The Associated Press contributed to this report.