The couple - who met at a boat party last summer following David's separation from wife Courteney Cox - have apparently decided to give their relationship another go after splitting last month.
A source told Life & Style magazine: ''David and Christina are fully back together.
''They spend all their time together. They are totally boyfriend and girlfriend again. David is happy and he says he loves her.''
A second insider added: ''Yes, it's true. They are back together.''
David - who has daughter Coco, seven, from his 11-year marriage to Courteney - went public with Christina last July when they appeared together at Comic Con, and later gushed: ''She's adorable. I'm not one to run around with different girls. I like someone that makes me feel good and that I can make feel good.''
But with the 'Scream' actor based in Los Angeles and Christina in New York, where she works as a reporter on 'Entertainment Tonight', the distance eventually got the better of them.
Announcing the split on twitter, David said: ''Sadly I'm single again. A long distance relationship proved to be too difficult.''