REUTERS/St. Louis County Prosecutor's Office
Darren Wilson, 28, was placed on administrative leave since the shooting on Aug. 9 in Ferguson, Missouri.
His resignation was announced Saturday by his attorney Neil Bruntrager.
A grand jury spent more than three months reviewing evidence in the case before declining in November to issue any charges against Wilson. He told jurors that he feared for his life when Brown hit him and reached for his gun.
For nearly four months, Ferguson has been plagued with violent protests that have stretched across the nation.
The US Justice Department is still conducting a civil rights investigation into the shooting and a separate probe of police department practices.