The 'You Da One' singer - who was seen leaving the 'Two and a Half Men' star's Hollywood home in the early hours of Wednesday morning (21.03.12) - is said to be smitten with Ashton after meeting him at a party of a mutual friend in December.
And friends says she is so taken with Ashton - who split from wife Demi Moore in November after he allegedly cheated on her - she is considering joining his Kabbalah group.
A source told The Sun newspaper: "The flirting began as soon as Rihanna and Ashton met and swapped numbers. 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.
"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. She's starting to enquire about Kabbalah sessions with his rabbi.
"She is curious about Kabbalah so has started asking questions. She's open to new things and has always been a very spiritual girl. If she and Ashton are getting really close, she wants to be able to understand it."
Ashton, 34, is keen to keep the romance as secret as possible as he doesn't want to upset Demi, who has recently completed a stint in rehab where she got treatment for exhaustion, substance abuse and an eating disorder.
An insider said: "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."
Rihanna, 24, was recently linked to her abusive ex-boyfriend Chris Brown.