Harington, who plays Jon Snow, and Leslie, Ygritte in the series, announced their engagement with an unassuming traditional ad in The Times newspaper.
The actors, who are both 30 years old, met on set in Iceland and have been together since 2012.
Leslie’s character is captured by Snow in season two and later becomes his on-screen lover.
Her line “You know nothing, Jon Snow” has become a popular quote from the series as a whole.
Ygritte is killed in a confrontation in season four.
“If you’re already attracted to someone and then they play your love interest in the show, it becomes very easy to fall in love,” Harington told Vogue magazine in an interview last year.
The couple were reported earlier this year to have bought a 15th-century home in eastern England with a moat, a swimming pool and a banqueting hall.
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