The Canadian singer-songwriter informed fans of the news on Instagram, revealing that Mod Sun, real name Derek Ryan Smith, had proposed while the couple were taking a boat ride in Paris on 27 March.
Lavigne shared a number of photos and videos from the day, as well as a picture of her diamond engagement ring.
“Oui! Je t’aime pour toujours. Dimanche. 27. Mars. 2022,” wrote Lavigne.
Mod Sun responded to the post, writing: “I love u so much! Forever n ever my angel.”
The couple began dating after collaborating on Lavigne’s recent album Love Sux.
Lavigne has been married twice before: to musicians Deryck Whibley, from 2006 to 2010, and Chad Kroeger, from 2013 to 2015.
Mod Sun also shared a series of images and clips from the Paris proposal with his Instagram followers.
“The day we met I knew you were the one. Together forever til our days are done,” he wrote.
“I had a dream where I proposed in pairs. I pulled out a ring + asked you to wear it. I was on one knee as I looked in your eyes. You’re too beautiful for my words to describe. I grabbed your hand + took one last breath... I said ‘will you marry me?’ + she said ‘yes’. I love you Avril”.