The couple is believed to have decided to give it another go after they were spotted indulging in some serious PDA at Family Guy creator Seth MacFarlane's house party in Los Angeles.
An onlooker told Life and Style magazine, 'They showed up together and made no effort to hide that they're back together.
'They were stealing kisses throughout the night, holding hands and just seemed to enjoy being together.
'Demi couldn't seem happier about it. They were dirty-dancing to the music, and when they were seated, Wilmer had his hands on Demi's knee the entire time. He was being very boyfriend-like and affectionate.'
The 19-year-old Skyscraper singer first hooked up with the former That 70s Show star back in 2010 before she was admitted to rehab for an eating disorder and emotional issues in November.