The 35-year-old star surprised everyone at her boyfriend Machine Gun Kelly's Indy 500 performance when she hit the stage and lit up with the rocker.
The brunette bombshell was met with cheers and applause from audience members during Barstool Sports' party at Grand Park in Westfield, Indiana on Friday.
Displaying her killer body in a tight black crop top, Fox flaunted her flat midriff and paired the look with jeans and heels.
Several fans caught the surprise moment on camera.
In one clip, Fox can be seen holding MGK's hand on the stage before he takes a cigarette and lights up.
The rocker is seen taking a puff before passing it to Megan who joins him in smoking.
She was then seen kissing her boyfriend's shoulder and dancing while he played one of his songs.
Meanwhile, Fox's estranged husband Brian Austin Green was 2,000 miles away taking care of their kids.
Over the weekend, the Beverly Hills, 90210 actor posted a picture of his girlfriend Sharna Burgess playing stepmom to his three sons --Noah, 8, Bodhi, 7, and Journey, 4 -- in Megan's absence.
"Pool days :) ❤️," Green captioned the shot showing his gf and kids in the pool together. "The best days ❤️," Burgess replied.
In the past, Fox has accused her estranged husband of "using" their children to make her look like a bad mom.
In October, she blasted her estranged husband on social media for posting a Halloween picture with their son Journey.
"Why does Journey have to be in this picture?" she commented on the post. "It's not hard to crop them out. Or choose photos that they aren't in. I had a great Halloween with them yesterday, and yet notice how absent they are from my social media."
"I know you love your kids. But I don't know why you can't stop using them to posture via Instagram," Fox added. "You're so intoxicated with feeding the pervasive narrative that I'm an absent mother, and you are the perennial, eternally dedicated dad of the year."
She ended her rant with one last dig.
"Congratulations, you truly are a remarkable human," she wrote. "Why do you need the internet to echo back to you what should be inexhaustibly evident in the way your children love you?"
Fox filed for divorce one month later after 10 years of marriage.