The 42-year-old actress and 44-year-old actor marked the occasion Sunday by dedicating sweet posts to the teenager on Instagram.
Witherspoon posted a photo of Ava posing outdoors, writing, "Happy Birthday to my girl @avaphillippe! You are the sunshine in my life! Love you to the moon and back!" Many fans remarked on Ava's resemblance to her mom.
"My goodness, she is definitely your spitting image @reesewitherspoon," one person wrote in the comments.
"At first glance I thought it was a throwback of Reese as a teen," another added.
Phillippe shared a picture of Ava on his own account, writing, "my little [star] on her birthday. love you, punk. such a proud dadish, i." Fans also commented on Ava's resemblance to her dad.
"Ava really is a perfect combo of the two of them. Here she looks like 90's Ryan P," one person wrote.
"She looks exactly like you Ryan," another said.
Witherspoon and Phillippe split in 2006 after seven years of marriage. The pair are also parents to 14-year-old son Deacon, while Witherspoon shares 5-year-old son Tennessee with Jim Toth and Phillippe is parent to 7-year-old daughter Kai with Alexis Knapp.
Witherspoon and Ava attended the Los Angeles premiere of A Wrinkle in Time together in February. The actress had enlisted Ava to star in a campaign for her company Draper James earlier in the month.
"There is nothing like the love I have for my daughter," she said on the Draper James website. "We share every emotion with each other, our hopes and dreams. We can talk for hours!"