The Fashion Police co-host joked on the microblogging site about Rihanna's revealing interview on Oprah Winfrey's OWN programme Next Chapter, where the 'We Found Love' singer shared that she and ex-boyfriend Chris Brown still "love each other".
"Rihanna confessed to Oprah Winfrey that she still loves Chris Brown. Idiot! Now it's MY turn to slap her," Rivers posted today (August 24).
Rihanna replied: "@Joan_Rivers wow u really do get slow when you're old huh?" and added: "Slap on some diapers."
Rihanna's interview with Winfrey - aired in the US on Sunday (August 19) - saw the singer discuss her family, her career and the aftermath of her relationship with Brown, which ended in 2009 when he assaulted her before the Grammy Awards.
Rivers previously poked fun at Taylor Swift's new single 'We Are Never Ever Getting Back Together', saying on Twitter: "It was inspired by Teddy Kennedy and Mary Jo Kopechne."
Watch Rihanna discuss Chris Brown on Oprah's Next Chapter below (US only):