George Zimmerman, the Florida neighbourhood watchman who shot dead an unarmed black 17-year-old male last year, has been found not guilty.
Lawyers for Mr Zimmerman, 29, argued he acted in self-defence and with justifiable use of deadly force in the death of Trayvon Martin.
The jury retired on Friday to consider its verdict on charges of either second-degree murder or manslaughter.
The case sparked a fierce debate about racial profiling in the US.
Spontaneous protest marches were staged overnight in US cities including San Francisco, Philadelphia, Chicago, Washington and Atlanta, US media reported.
Police and community leaders appealed for calm in Sanford, the Florida town where the shooting took place.
'No further business'
After the verdict, Judge Deborah Nelson told Mr Zimmerman he was free to go.
"Your bond will be released. Your GPS monitor will be cut off when you exit the courtroom over here. And you have no further business with the court," Judge Nelson said.
Mr Zimmerman showed little reaction as the verdict was read out.
"Hopefully everyone will respect the jury's verdict," his lawyer Mark O'Mara told reporters after the case.
He said Mr Zimmerman would now have to be "very cautious and protective of his safety because there is still a fringe element who have said that they would revenge - that they would not listen to - a verdict of not guilty."
Another member of his defence team, Don West, said: "I'm thrilled that this jury kept this tragedy from becoming a travesty."
The BBC's David Willis, in Florida, says the case brought into sharp relief some of the most divisive issues in the United States: race, gun control and equal justice under the law.
Florida police did not arrest Mr Zimmerman for six weeks after the shooting, provoking mass rallies in Florida and throughout the US.
Police justified their decision not to detain him by citing the state's controversial "stand your ground" law, which allows a citizen to use lethal force if he or she feels in imminent danger. Police initially said the law prevented them from bringing charges.
Benjamin Crump, the Martin family lawyer, said: "Trayvon Martin will forever remain in the annals of history... as a symbol for the fight for equal justice for all."
He, too, appealed for calm, saying "for Trayvon to rest in peace, we must all be peaceful".
The family's legal team said they were not in the courtroom when the verdict was read out.
State Attorney Angela Corey said she believed prosecutors had "brought out the truth on behalf of Trayvon Martin".
"This case has never been about race or the right to bear arms,'' she said.
"We believe this case all along was about boundaries, and George Zimmerman exceeded those boundaries."
As the jury retired on Friday, the judge told the panel of six women to consider whether Mr Zimmerman acted in self-defence and with justifiable use of deadly force.
Without explicitly discussing race, the prosecution had suggested Mr Zimmerman assumed the African-American teenager, who was wearing a hooded sweatshirt as he walked in the rain, was up to no good.
But the defence said Trayvon Martin punched their client, slammed his head into the pavement and reached for Mr Zimmerman's gun.
The accused, who was legally armed with a pistol, had been sitting in his vehicle on a dark street when he saw Martin.
Mr Zimmerman telephoned police to report a suspicious person, then left his vehicle in apparent pursuit of the teenager.
Shortly afterwards, Martin was found dead, shot in the chest.
'Civil rights violations'
Earlier, Mr Zimmerman's lawyer said he had proven his client's "pure, unadulterated innocence" in Martin's death.
But prosecutors said the accused had told a series of lies.
US President Barack Obama commented on the case last March following calls for the arrest of Mr Zimmerman.
He said the "tragedy" of an unarmed black teenager shot dead in Florida should prompt some national soul-searching.
"If I had a son, he would look like Trayvon," President Obama told reporters at the White House.
Following the verdict, the National Association for the Advancement of Colored People (NAACP) - best known for its battles against segregation and discrimination - said it was calling "immediately for the Justice Department to conduct an investigation into the civil rights violations committed against Trayvon Martin".
"This case has re-energised the movement to end racial profiling in the United States," Roslyn M Brock, chairwoman of the NAACP, said in a statement.