Ferguson, Missouri policeman Darren Wilson said he was stepping down due to safety concerns, according to the letter sent to the police department and published by local daily the St Louis Post-Dispatch.
"It was my hope to continue in police work, but the safety of other police officers and the community are of paramount importance to me," the letter from the 28-year-old said.
Wilson's lawyer had previously indicated that he would resign from the police force and would never work as a police officer again.
Wilson shot and killed black teenager Michael Brown in August, claiming he acted in self-defence.
The shooting prompted racially-charged protests that erupted again last week and spread across the nation after a grand jury decided not to charge Wilson in the shooting.
Protesters claim US police put a higher value on white as opposed to black lives.
"I have been told that my continued employment may put the residents and police officers of the City of Ferguson at risk, which is a circumstance that I cannot allow," Wilson's statement said.
"It is my hope that my resignation will allow the community to heal."
