"Happy birthday to my ride or die," she wrote, including devil and hear emojis. "I love you I love you I love you!! I’m so lucky to have such a bad—s b— by my side." She also included the hashtag #20.
The dark photo was taken by Berg in a bathroom, with the two posing in front of a mirror.
Berg is frequently seen in Winter's Instagram photos. Berg also posted a photo of the two friends having dinner on Jan. 12.
On Thursday, Winter wished another friend, Henry Danger actor Joe Kaprielian, a happy 20th birthday. The accompanying photo also showed Winter in a racy crop-top, with pink hair. The hairdo led fans to think it was taken during Coachella, since Winter now has short, black hair.
Winter herself turned 20 on Sunday. She has been dating fellow actor Levi Meaden for over a year.
The Modern Family actress has had an exciting year so far, and it has nothing to do with awards for the long-running ABC sitcom. On Jan. 7, she tweeted about arriving at an Airbnb she was renting, only to see it see it covered in dog poop.
"I come back from lunch to find shit thrown all over the front of the house I’m staying in. I will find you with these cameras and you are f—," she wrote. "I still can’t believe someone threw [poop] all over my Airbnb...like bruh I just got here with my scrabble chiiiiiiillllllll. Never been happier to go home."
She later thanked Meaden for being by her side during the situation. He "is my rock and I couldn’t be luckier," she wrote. "I love you."
Photo credit: Instagram / @Ariel Winter