The Hairspray star, who was arrested in New York City on Thursday after police allegedly found drugs at her home, fired off a tweet mocking the Umbrella hitmaker's appearance.
She stated: "Rihanna you look so ugly ..."
Bynes then referenced the singer's infamous 2009 fight with ex-boyfriend Chris Brown, which left her bloodied and bruised, adding: "Chris brown beat you because you're not pretty enough."
It is unknown if the Bajan singer saw the tweets, but she later wrote on the site: "Ya see what happens when they cancel Intervention?" - a nod to a recently cancelled US show which helps addicts address their issues.
However, Bynes shot back at the message, writing: "unlike ur fugly faced self I don't do drugs! U need the intervention ..."
The tweets by Bynes have since been deleted.
The 27-year-old's messages came just a day after she took to the site to claim she was mistreated by one of the officers who arrested her on Thursday. Police have launched an investigation into the allegations.
Bynes was charged on suspicion of marijuana possession, attempted tampering with physical evidence, and reckless endangerment - after allegedly throwing a bong out of the window of her 36th-storey apartment in Manhattan.