"Happy birthday to my soulmate, the love of my life, my favourite person in the world.....my angel," he wrote, sharing a series of photos of the two of them together.
In one, the Canadian icon is wearing what looks like a Vancouver Canucks jersey. Another features them both in very punk rock suits at the 2021 MTV Video Music Awards, with Avril wearing a pink plaid number. The rest see them cuddling up to each other on the beach, in the car and at industry events.
"To the most radiant + graceful woman I've ever laid eyes on, you have made my dreams come true in so many ways," he continued. "You are my calm in the chaos + my shelter from the storm. You are elegance + rock n roll wrapped up in one body.
"I was born without you, dying to find you, I'll always remind you.....I love you with all my heart @avrillavigne+ the best is yet to come. Happy birthday my little icon."
He added a green heart emoji at the end.
Aww! Avril was very touched by the message.
"The sweetest most beautiful words ever," she replied in the comments. "So glad I get to spend it with you my sunshine. 'I la la la la la la la la la la love you.'"
Avril and Mod Sun went public as a couple earlier this year when they made their red carpet debut at the aforementioned MTV Video Music Awards. The Canadian pop music icon was rumoured to be dating the rapper back in February, and those whispers went up a notch when the two were seen out with Megan Fox and hitmaker Machine Gun Kelly the next month.
Avril was previously married to Sum 41 singer/guitarist Deryck Whibley from 2006 to 2010 and Nickelback frontman Chad Kroeger from 2013 to 2015. She also dated The Hills' Brody Jenner from 2010 to 2012.
Happy Birthday, Avril!