The singer vented anger after a white neighbourhood watch member was cleared of killing unarmed 17-year-old Trayvon Martin.
She took to Instagram after George Zimmerman walked free over the shooting ranting that the US criminal justice system was "phucked".
The 25-year-old singer spoke out on the matter the morning after Zimmerman, 29, was found not guilty.
Posting a photo of Trayvon on Instagram, Rihanna wrote: "A child was gunned down for no reason! And nothing about that sounds like murder? My baby brother is 17, this rocked my whole s***! #thesystemisandhasalwaysbeenphucked"
On Twitter she also said: "This is the saddest news ever!!! #whatsjustice #pray4theMartinFamily"
She added: “Only in America #TrayvonMartin”
The pop star then urged her 30MILLION followers to sign a petition to the Department of Justice.
She also criticised the American justice system saying: “A system cannot fail those it was never built to protect #JudicialFail”
Outrage poured from street demonstrations as protesters called for justice for the victim.
In New York City, hundreds of protesters marched into Times Square on Sunday night, zigzagging through the streets to avoid police lines.
Sign-carrying marchers thronged the busy intersection, chanting “Justice for Trayvon Martin!”
Beyonce Knowles' sister Solange held a rally for Trayvon to speak out against the Zimmerman verdict on Sunday afternoon.
World heavyweight boxing champion Lennox Lewis also took to Twitter to express anger at the verdict.
He said: “Let’s hope this doesn’t lead to the deaths of more black children. How u respond is most important right now.”
George Zimmerman said he was defending himself and was cleared of second degree murder and manslaughter at a court in Florida on Saturday.