The 30-year-old "Gossip Girl" actor and Rittenhouse, 26, were seen getting close to each other during his 30th birthday party in California, on Saturday, reported Aceshowbiz.
"Rebecca was by Chace's side most of the night. I wouldn't call them official... But there is a romance between them for sure," a source said.
The co-stars have been frequently spotted hanging out together.
"He and Rebecca have been hanging out... Although there is nothing serious... They had great chemistry on set and he really enjoys hanging out with her," the source said.