"Y'all ready for summer to end? I'm definitely not!" the singer wrote under the photo, which is from the same set as the image used on a 2008 OK! cover. (In it, a blonde, tanned Spears, now 30, rocks a bright white bikini, huge sunglasses, and a massive smile.)
Fans began to suspect something was up when Spears made no mention of when or where exactly the sun-soaked image was taken.
Regardless, the X Factor beauty managed to prove she's still got it in a separate picture posted Tuesday. In that shot, Spears, wearing a stunning little black dress, poses in front of a flower bed for a Happy birthday message to one of her family members.