Kunis, 28, locked lips with Kutcher during an afternoon party last week, reports usmagazine.com.
"They were seated on a couch on the same side of the table and Mila was running her hands through Ashton's hair. She had her elbow resting on his back and couldn't stop playing with his hair and rubbing his shoulders," said an eyewitness.
Kutcher recently split from his estranged wife Demi Moore after he was found cheating on her after six years of married life.
Kunis was in a relationship with actor Macaulay Culkin, but parted ways in 2011.