A former San Francisco police officer is facing charges in the homicide of an unarmed carjacking suspect he gunned down just days after he was hired, prosecutors said.

Christopher Samayoa, who was working his fourth day as a cop when he fatally shot Keita O’Neil in 2017, has been charged with voluntary manslaughter, involuntary manslaughter, assault by a police officer and related counts, San Francisco District Attorney Chesa Boudin said Monday.

The case is believed to be the first homicide prosecution against a law enforcement officer in the city’s history, Boudin said.

“For too long, we have seen the failures of our legal system to hold police accountable for the violence committed against the members of the public they are entrusted to keep safe,” Boudin said in a statement. “In my administration, police officers are not above the law.”

A judge signed a warrant for Samayoa’s arrest on Saturday. He’s expected to surrender later this week, Boudin said.

The rookie officer, who was later fired, was in the passenger seat of a patrol car with a training officer following O’Neil, who was suspected of carjacking a California State Lottery van on Dec. 1, 2017, prosecutors said.

The van reached a dead-end street and O’Neil hopped out of the vehicle, prompting Samayoa to open fire through the passenger window as the suspect ran past the officer.

“Mr. O’Neil has no weapons, he was unarmed,” Boudin told reporters at a press conference, KPIX reported. “Body camera footage from other officers shows that not a single other officer pulled out their service weapon or pointed it at Mr. O’Neil. As a result of Officer Samayoa’s terrible, tragic and unlawful decision that day … Mr. O’Neil was killed.”

O’Neil, who was shot near his collarbone, was later pronounced dead at a hospital. His cause of death was ruled to be a homicide, authorities said.

Relatives of O’Neil, whose mother has filed a civil rights lawsuit in the slaying, have been informed of the charges facing the former cop.

“I am happy to hear this news, and hoping it brings some justice to our family,” O’Neil’s aunt, April Green, said in a statement.

It’s unclear if Samayoa has hired an attorney, but the San Francisco Police Officers Association said it intends to back the onetime officer.

“We are committed to ensuring that Christopher and his family are supported during this difficult time and that he is accorded his due process rights and provided with a vigorous defense against these charges,” the organization’s president, Tony Montoya, said in a statement Monday.

With Post wires