"Happy Birthday to my girl @avaphillippe! You are the sunshine in my life! Love you to the moon and back!" the "Big Little Lies" star, 42, wrote next to an Instagram pic of Ava, who's the spitting image of her Oscar-winning mom.
In the comments section, fans gushed over the pair's uncanny likeness.
"She is your twin!" wrote one.
"Wow, I thought that was a picture of you when you were young," wrote another.
Last year, Witherspoon revealed that Ava's resemblance to her has caused confusion, especially among her fans. "People come up to her (all the time). We were at a premiere and they were congratulating her on her performance," the actress shared with E! News. "She's like, 'I'm not in the movie.'"
But when Ava's dad and Witherspoon's ex-hubby, Ryan Phillippe, took to Instagram to share his own birthday message to his "little star," fans responded by marveling over how Ava favors him.
"She looks exactly like you, Ryan," one wrote.
"If you look closer and compare her to young Ryan, you’d notice they’re literally twins," shared another.
It's a subject we here at TODAY.com have debated ourselves: Which of her famous parents does Ava most resemble? We even created a poll about it to have readers chime in with their opinion.
For the record, 48 percent of the poll's voters said Ava is Mom's Mini-Me, while 43 percent said the teen looks just like dear old dad.
But we still see a lot of both in the pretty teen.
Perhaps one commenter under Ryan Phillippe's sweet birthday message summed it up best: "Ava really is a perfect combo of the two of them."
We couldn't agree more!