The 'Firework' hitmaker - who spent the festive period in Hawaii with friends - has shared a number of heart-to-heart phone calls with her pal as she deals with her split from British comic Russell Brand after 14 months of marriage and the 'We Found Love' singer has been a "great support" in helping her cope.
A source told The Sun newspaper: "Katy is hardly speaking to anyone right now, but she called Rihanna as soon as she heard Russell was filing for divorce.
"Katy was embarrassed about calling her parents as they'd been so against Russell in the first place, but Rihanna was a great support for her and she really helped to calm her down."
Rihanna is now said to be planning a holiday to Mexico with Katy to help her get over Russell.
An insider told the Daily Mirror newspaper: ""Rihanna has been there for Katy, texting her from Miami where she's on holiday. Now they are planning a girlie getaway so Katy can clear her head."
While Russell - who is rumoured to be moving back to the UK following the split from Katy - is entitled to half of the singer's estimated $70 million fortune under Californian law, the pair have vowed to part as "friends" and want the split to remain amicable.
The former couple experienced problems very soon after getting married, as they rowed constantly and rarely saw one another due to their hectic schedules.
The main point of contention is said to have been former drug addict Russell's hatred of the party scene, which led to many arguments when he wanted to leave nights out early and his spouse didn't.
However, insiders sources there is no animosity between them, with one telling website TMZ: "It was just a bad pairing."