Balotelli, 23, cut a lost and frustrated figure as Italy went down 1-0 to Uruguay on Tuesday to end their interest in the World Cup.
Coach Cesare Prandelli hauled off an ineffective Balotelli, who scored the winner in the Azzurri's opening match against England, at half-time against Uruguay.
A social media video posting on Instagram by an Italian said: "Mario, do you know what the thing is? You're not Italian, get out."
As a rare Italy international of African origin -- his birth parents are Ghanaian -- Balotelli stands out in an Italy shirt.
Balotelli replied to the video post and sent a link to his impassioned defence on his own Twitter page.
"I'm Mario Balotelli, I'm 23 and I didn't choose to be Italian," he said.
"I really wanted it because I was born in ITALY and I've always lived in ITALY. This World Cup meant a lot to me and I'm sad, angry and disappointed with myself.
"It's true maybe I could have scored against Costa Rica (they lost 1-0 last Friday), you're (in the plural) right, but then what?
"Then what's the problem? Maybe what you all want to say is this?
"Don't put all the blame only on me this time because Mario Balotelli gave everything for the national team and didn't mess up anything (from a character point of view), so look for another excuse because Mario Balotelli has a clear conscience and is ready to go forward stronger than before and with his head held high.
"Proud to have given his all for His country. Or maybe, as you say, I'm not Italian. Africans would never abandon one of their brothers. NEVER.
"In that respect us blacks, as you call us, are light years ahead. SHAME is not that you can miss a goal or run less or more. SHAMEFULNESS ARE THESE THINGS. True Italians! True?"
It is not the first time Balotelli has been targeted by racist Italian supporters.
The AC Milan striker has several times faced racial abuse on at football grounds in Italy, most notably from Juventus fans during his Inter Milan days, before heading to England and Manchester City.
He was even racially abused at an Italy training camp in Coverciano, just outside Florence, in May in the build up to the World Cup.
In 2010 in a friendly against Romania in Austria he was abused by Romanian fans while some Italian supporters held up a banner saying: "no to a multi-cultural Italy."
