The 'Keeping up with the Kardashians' star only filed for divorce from basketball star Lamar Odom yesterday (13.12.13), but is already said to have moved on with the baseball player.
Khloe, 29, and Matt were spotted going to the gym in Los Angeles, with Matt driving his black Jaguar car and Khloe following in her Range Rover. The stars then both walked in taking separate entrances with Khloe covering her face.
A source told website HollywoodLife.com: ''Khloe and Matt are dating, but it's not serious yet. But Khloe's already acting like she's in love ... Khloe's like that, she loves hard, but she moves on quick.''
He wrote underneath: ''Best time of the year!! It's gonna b a Cali Christmas this year (sic)''
Khloe cited ''irreconcilable differences'' in papers to end her four-year marriage to Lamar, following rumours of him using dugs and cheating on her with other women. They had originally separated in September, but he had begged her to take him back and has been left ''deeply saddened'' by her decision to divorce him.
A source said: ''He knew it was coming and it was only a matter of time. He felt like he gave his all to try and save the marriage, but ultimately he wasn't able to.
''There will always be a place in his heart for Khloe. He will always love her and her family. She has decided and there is nothing more he can do. He has to accept that.''
Many ticket-holders couldn't get into the O2 Arena show on Tuesday night (September 19th) because they didn't bring photo ID to match their booking.
An album re-release, a new song and a documentary mark the singer's legacy this year.
The actor plays the titular hero in the forthcoming adaptation.