The Hairspray star, who was arrested in New York City on Thursday (23May13) after cops allegedly found drugs at her home, took to her page on the site to fire off a tweet mocking the Umbrella hitmaker's appearance.
She stated, "Rihanna you look so ugly tryin (sic) to be white."
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."
She continued, "Rihanna no one wants to be your lover so you call everyone and their mother that I almost named my new dog Rihanna."
It is unknown if the Bajan beauty saw the tweets, but she later wrote on the site, "Ya see what happens when they cancel Intervention?" - a nod to recently cancelled U.S. show Intervention, which helps addicts address their issues.
However, Bynes shot back at the message, writing, "unlike ur (your) fugly (f**king ugly) faced self I don't do drugs! U need the intervention dog! I met ur ugly face in person! U aren't pretty u know it!"
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 cops 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.