and Nickleback frontman Chad Kroeger are engaged to be married, Lavigne's rep told ABC News.
The Canadian rockers kept their relationship so quiet that hardly anyone knew they were even dating.
Following a six-month courtship, Kroeger proposed with a 14-carat diamond ring on Aug. 8, according to People magazine.
Lavigne, 27, first met Kroeger, 37, when they got together in February to write a song for Lavigne's upcoming fifth studio album.
"A romantic relationship blossomed as they spent time writing together," a friend of Lavigne told People.
"He makes her so happy," a family source told the magazine. "Both of their families could not be more excited."
This will be Kroeger's first marriage and the second for Lavigne, who split from Sum 41 frontman Deryck Whibley in 2009.
She also dated Brody Jenner.