A Rikers Island inmate was found dead in his cell Saturday — the sixth person in custody to die this year in New York City’s jail system, sources said.

The inmate, who was described by jailhouse sources as 20 or 21 years old, was found unresponsive with blood coming from his nose inside the Robert N. Davoren Center, jailhouse sources said.

It wasn’t clear how long the man had been dead, but his death does not appear to be a suicide, according to sources.

Medical staff found no pulse on the inmate and left him there until a medical examiner arrived.

The man had been charged with murder in February and robbery and was due back in court this week, according to records.

The sudden death shook some staffers at the center, where young adults are housed, sources said. It was the inmate’s first stint in jail, one source told The Post.

“He was in his cell, blood coming out of his nose. We don’t know how he died,” the source said. “He was just a kid.”

The latest comes in the wake damning federal report that found the conditions in the troubled city jail systems to be “severe and potentially life-threatening.”

The last inmate to die in custody was 31-year-old Mary Yehudah, who was found unresponsive in her cell on May 18. She died just 10 days after a 25-year-old inmate hanged himself.

Last year, 16 people died in city lockups — the most since 2013.