"That moved on to texts and arranging to meet. They snuck off to a Santa Monica hotel a few weeks ago.
"They are two single people having a fling, so thought it would be best to meet in secret," the insider continued.
Rihanna has also supposedly been asking Kutcher about his belief in Kabbalah, a Jewish religious teaching.
"Rihanna has told friends he's funny and cute. Both of them love a good time and think they're too busy for anything serious just yet," the informant added. "She's starting to enquire about Kabbalah sessions with his rabbi."
Referring to Kutcher's ex-wife Demi Moore, the source added: "Ashton does not want to rub a new relationship in her face. She's still vulnerable. He's a good guy and, with all Demi's recent health problems, people have forgotten that."