The couple is said to have met through common friends and since then have been happy together, reported Us magazine.
"Marloes and Alex are very much a couple. They met through friends and have been together seriously for a few weeks. It's early days, but they are really happy," a source said.
The 24-year-old actor, who celebrated his birthday yesterday, visited Disneyland in Anaheim, California, with his lady love and a group of friends.
"Happy birthday baby Alex Pettyfer #birthdayboy," Horst, 25, captioned a photo.
Horst has modeled for Victoria's Secret, H&M, and DKNY. Earlier this week, she was named the new face of Maybelline.