The pair are alleged to have met and spent the night together while Ora was in New York.
According to Us Weekly, Ora's recent break-up - which has been plagued by rumours of infidelity after Kardashian posted a series of angry tweets - was a result of her hook-up with Hill.
The 'Hot Right Now' singer and Get Him to the Greek star - who split from Dustin Hoffman's daughter Ali in September - are said to have clicked straight away
"Rita spent the night with Jonah during a trip to New York," an apparent source said. "Then Jonah bragged about it to friends, Rob found out and Rita confessed! That was the last straw."
Ora has previously claimed that she and Kardashian separated because they could not maintain a long-distance relationship. The singer was recently forced to cancel a concert in Philadelphia due to illness.
Watch the video for Rita Ora's latest single 'Radioactive' below: