Darren Wilson insists he acted in accordance with the law
US police officer Darren Wilson - who fatally shot unarmed black teenager Michael Brown, triggering nationwide riots - has resigned, his lawyer says.
Neil Bruntrager said his client had been on administrative leave since the 9 August shooting in Ferguson.
The Missouri town and other US cities saw several days of rioting and looting this week after a jury decided not to bring charges over the killing.
Mr Wilson, who is white, said he had acted in accordance with the law.
Mr Bruntrager made the announcement on Saturday, saying the resignation was effective immediately, according to the Associated Press.
An undated family snapshot of Michael Brown
Demonstrators flooded on to the streets of Ferguson after the verdict, with several buildings set alight
It was the latest bout of rioting since the August shooting
He declined to provide any further comments, but said more details would be released shortly.
Earlier this week, Mr Wilson told US media that before the shooting, Mr Brown had pushed him back into his car, hit him and grabbed at his drawn gun, and said that he felt "like a five-year-old holding on to [US wrestler] Hulk Hogan".
The policeman said he had feared for his life.
Mr Brown's supporters said he was attempting to surrender to the policeman when he was shot. Some witnesses said the teenager, who was unarmed, had his hands up.
However, the state prosecutor said physical evidence had contradicted some of the witness statements.
The family of Mr Brown, who was 18, have said they felt "crushed" by the decision.
Their son was killed after being shot six or seven times.
Many in the African-American community had called for Mr Wilson to be charged with murder, but after three months of deliberation a Missouri grand jury - of nine white and three black members - made no recommendation of charges.
The decision means Mr Wilson will not face state criminal charges over the shooting.
It also triggered a nationwide debate over relations between black communities and law enforcement.
Protests were held in a number of US cities, including New York, Washington and Los Angeles.
More than 100 people were arrested during the demonstrations.
The US justice department has also launched a federal investigation into whether Mr Wilson violated Mr Brown's civil rights.