The Spider-Man 3 star, 31, met the Melrose Place actress, 22, at a friend's Hollywood party in August (09) and the couple has been spotted out on various dinner dates ever since.
An eyewitness tells the National Enquirer, "They could not keep their hands off each other. They sat side by side and were inseparable.
"Topher kept kissing Katie and rubbing her shoulders. He looked in love."
Grace has previously been romantically linked to Ivanka Trump and actress Teresa Palmer.