Kyle Rittenhouse, 18, killed the men and wounded a third on the streets of Kenosha, Wisconsin, on 25 August 2020.
During the high profile and politically divisive trial, his defence said he had feared for his life. Prosecutors argued he was looking for trouble that night.
National Guard troops have been sent to the city amid fears of unrest.
Mr Rittenhouse faced five charges, including intentional homicide, which carries a life sentence.
His fate was decided by a 12-person jury composed of seven women and five men, who had spent more than three days deliberating.
Two nights before he turned up in Kenosha last year, riots had erupted on its streets after police shot Jacob Blake, a black man, leaving him paralysed.
Mr Rittenhouse, then aged 17, had travelled to the city from his home in Illinois and, armed with a semi-automatic rifle, he said he sought to help protect property from unrest.
The case has become a flashpoint in the highly polarized debate over guns rights in the US.
Mr Rittenhouse is championed as a hero by those who say he tried to keep the peace in sometimes violent protests.
Others were horrified by what they saw as a heavily armed teenage vigilante in a volatile setting.
BBC correspondent Nomia Iqbal, who was at the courthouse on Friday, said several cars drove past tooting their horns and shouting “Free Kyle” and “We love the second amendment” after the verdict was announced.
On the court steps Jacob Blake’s uncle was in tears and said he was shocked at the verdict. He said that if Mr Rittenhouse had been a black teenager “police would have shot him dead.”
During the trial, jurors were shown video, sometimes frame by frame, leading up to and after each shooting.
Armed with the semi-automatic rifle, Mr Rittenhouse fatally shot Joseph Rosenbaum, 36, and Anthony Huber, 26, and then wounded 27-year-old Gaige Grosskreutz. Mr Rittenhouse and the men he shot are all white.
In their closing arguments, his lawyers argued that he was “trying to help this community” and “reacted to people attacking him”.
Prosecutors, meanwhile, questioned why Mr Rittenhouse broke curfew in a city he did not live in and “pretended to guard” people and property he was not familiar with.
“You cannot claim self-defence against a danger that you create,” they said.
Wisconsin Lieutenant Governor Mandela Barnes reacted angrily to Friday’s verdict.
“Over the last few weeks, many dreaded the outcome we just witnessed,” he was quoted by the Associated Press news agency as saying.
“The presumption of innocence until proven guilty is what we should expect from our judicial system, but that standard is not always applied equally. We have seen so many black and brown youth killed, only to be put on trial posthumously, while the innocence of Kyle Rittenhouse was virtually demanded by the judge.”