Watson met Simmons on the set of upcoming movie The Perks of Being a Wallflower. Watson had previously denied that they were dating after rumours of a romance emerged in June.
Photos show Watson and the 24-year-old during a lunch date in Santa Monica, California sharing an passionate embrace.
"They arrived outside the restaurant and they immediately shared a very tender kiss," a witness told the Sunday Mirror.
"They weren't at all shy about it and obviously didn't mind who saw. Emma reached up to kiss him and gazed at him adoringly before he pulled her in for a tender embrace.
"They spent a few seconds staring into each other's eyes and it seemed obvious they were a couple. After one or two minutes they walked into the restaurant and had lunch."
Emma Watson recently revealed that she "can't wait" to become a mother in the future.