The couple, who are parents to a 9-month-old son Dakota, got engaged after four years together, a source told PEOPLE on Wednesday.
Song, 33, was recently photographed sporting a diamond ring on her left hand while out and about in Beverly Hills, California, on Monday.
The Dollface actress and Culkin, 41, met on the set of their movie Changeland in Thailand and were first spotted together in July 2017.
While the two actors are notable stars, they've kept their relationship private on social media, except for special occasions, like when Song posted a photo of them on a date night in November at an L.A. Rams game.
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"Very few things can get us out of the house and away from our son," she wrote in the caption.
Song also wished Culkin a happy 40th birthday in a post on August 2020. The photo showed them wearing matching plaid pj's with their respective names on them.
"Happy 40th birthday to this magical being," she wrote in the caption. "I could sit here and write endlessly about how wonderful, kind, loving, genuine, loyal, honest, brilliant and hilarious you are, and how grateful I am that I get to share and do this life thing with you."
"But firstly, that would take me an eternity and secondly, you won't even see this because you don't ever use instagram. Hahaha," she added. "My unicorn that I never thought could exist, I am luckiest person in the world because I am loved by you."
Backgrid Brenda Song and Macaulay Culkin
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The couple welcomed their son, Dakota, in April of last year naming him after Culkin's late sister Dakota.
"We're overjoyed," Culkin and Song said in a brief statement about the arrival of their first child.
In 2018, Culkin made rare comments about his relationship with Song and their plans for the future while on The Joe Rogan Experience podcast where he revealed he and Song were trying to have a baby.
"This one's a good one, so I'm probably going to put some babies in her in a little bit," Culkin said, laughing. "I mean, we've definitely been practicing."
He continued, saying he and Song have "a good life."
"I have a pretty little family — a pretty girl, a pretty dog, a pretty cat, and all that stuff. We're gonna move," he said at the time. "We're doing the house thing and all that kind of stuff."
Dimitrios Kambouris/WireImage; Barry King/Getty Macaulay Culkin (L); Brenda Song
When the Home Alone star appeared on the cover of Esquire in February 2020, Song told the magazine what drew her to her fiancé.
"People don't realize how incredibly kind and loyal and sweet and smart he is," said Song. "Truly what makes Mack so special is that he is so unapologetically Mack."
She continued: "He knows who he is, and he's 100 percent okay with that. And that to me is an incredibly sexy quality."