The duo, who have previously denied rumours that they are romantically involved, attended the wrap party of Jobs in Los Angeles last week.
A source has now told People that they "showed up as a couple and left as a couple".
"[They] appeared to be lighthearted and in good spirits, and hardly left each other's side," the insider added. "They stayed within five feet of each other practically the entire evening. It looked like young love."
Speculation surrounding Kutcher and Kunis's relationship has intensified recently after they were reportedly seen holding hands at lunch and kissing at a party.
Kutcher's estranged wife Demi Moore is rumoured to be "devastated" about his alleged romance with Kunis.