"A sentence of death is the ultimate, permanent punishment," Greitens said in a statement. "To carry out the death penalty, the people of Missouri must have confidence in the judgment of guilt. In light of new information, I am appointing a Board of Inquiry in this case."
The execution of Marcellus Williams, 48, had been scheduled for Tuesday evening.
His attorneys said that DNA evidence unavailable during his 2001 trial proved his innocence.
However, the Missouri Attorney General's Office had argued the execution should be carried out, saying the DNA evidence doesn't overcome non-DNA evidence that connects the inmate to the crime.
In a statement, Greitens announced the creation of a new five-person Board of Inquiry. The board, which will have subpoena power, will review evidence, in addition to newly discovered evidence, and offer a recommendation to the governor, who will determine whether Williams is granted clemency.
A spokesman for the governor said the stay will remain in place as long it's necessary for the case's review and for the governor to make a final decision.
Greitens' decision Tuesday was praised by the Innocence Project, which assisted Williams' lawyers in asking the governor to convene the board.
"We are relieved and grateful that Gov. Greitens halted Missouri's rush to execution and appointed a Board of Inquiry to hear the new DNA and other evidence supporting Mr. Williams' innocence," said Nina Morrison, senior staff attorney at the Innocence Project.
"While many Americans hold different views on the death penalty, there is an overwhelming consensus that those sentenced to death should be given due process and a full hearing on all their claims before an execution, and the governor's action honors that principle."
The Death Penalty Information Center, a Washington, DC nonprofit,, said the governor's decision to stay Williams' execution is "an important step in ensuring that Missouri does not execute an innocent man."
Defense attorney Larry Komp that he was "ecstatic" upon hearing about the stay, and added that he thought Williams felt similarly.
"He was thoughtful and I believe happy, and asked where do we go from here," Komp said. "His reaction was the same as mine: Happy for 30 seconds and then 'Alright, let's get to work.'"
In a statement, St. Louis County Prosecutor Robert P. McCulloch said he is "confident that any Board and the Governor, after a full review of all evidence and information, will reach the same conclusion" that the jury and several courts have reached over the past 20 years.
Lawyers: His DNA isn't on murder weapon
Williams was convicted in the death of Felicia Gayle, 42, a former reporter for the St. Louis Post-Dispatch newspaper who was stabbed 43 times inside her home in August 1998.
The newly acquired evidence shows Williams' DNA was not found on the murder weapon, Williams' lawyers say, though DNA from another male was found.
That evidence was not available when Williams went to trial in 2001, court documents say. Williams maintains his innocence and says he was convicted on the testimony of individuals who were, themselves, convicted felons.
Forensic DNA expert and biologist Greg Hampikian, who was hired by defense lawyers, told CNN on Monday that "when you're stabbing, DNA transfers because of restriction and force. If you're stabbing anyone, you have a good chance of transferring your DNA because of that force."
The analysis of DNA on the knife "isn't enough to incriminate someone, but it is enough to exclude somebody," he said. "It's like finding a Social Security card with some blurred numbers. There's still enough there to at least exclude someone."
Hair samples found at the crime scene don't match Williams' DNA, Hampikian said. A footprint found at the scene also does not match the defendant's shoes, his lawyers said.
But the state attorney general's office, in addressing the new DNA evidence in court documents filed in federal court last week in opposition to a stay of execution, offered a possible explanation of why none of Williams' DNA was found on the knife.
State confident in his guilt
The new DNA evidence "does not come close to showing Williams is actually innocent," the documents state. "It would be unsurprising if Williams, who wore a coat from the crime scene to cover his bloody shirt, wore gloves when he committed the burglary and the murder," the documents say.
The office said Williams' guilt was proven without DNA evidence.
"Based on the other, non-DNA, evidence in this case, our office is confident in Marcellus Williams' guilt," said Loree Anne Paradise, deputy chief of staff for Attorney General Josh Hawley.
The non-DNA evidence includes a laptop belonging to the victim's husband, which Williams sold and police recovered, and some of the victim's personal items, which police found in the trunk of the car Williams drove, according to court documents.
Williams got picked up about three weeks after Gayle was killed on unrelated charges. His cellmate from that time at a local jail, Henry Cole, and Laura Asaro, Williams' girlfriend, testified for the state, saying Williams told them separately that he committed the murder, according to the documents filed by the state attorney general.
Rejected by state Supreme Court
Williams' defense filed a brief Monday night with Supreme Court Associate Justice Neil Gorsuch. The governor's stay means that the justices won't act on the issue for now.
The Missouri Supreme Court on August 15 turned down defense lawyers' bid to have the execution stopped and didn't provide a reason, according to the St. Louis Post-Dispatch
The case has drawn high-profile advocates. Sister Helen Prejean, the Roman Catholic nun who fights the death penalty and was featured in the movie "Dead Man Walking," is involved in the case. Amnesty International is urging Gov. Eric Greitens, a Republican, to grant clemency.
"It's gaining public attention," Amnesty International researcher Rob Freer said of the case. "I think there is a heightened sensitivity that it is now proven that the capital justice system" has errors, he said.
Samuel Spital, the director of Litigation at the NAACP Legal Defense & Educational Fund, said the lack of physical evidence linking Williams to the murder is "not the only disturbing aspect" of the case.
The trial prosecutor was allowed to preemptively strike six out of seven prospective black jurors, Spital told CNN, noting that Williams is black and Gayle was white.
"The prosecutor offered as a race 'neutral' explanation for one of the strikes that the black juror looked like Mr. Williams and that the juror worked for the Post Office, even though the same prosecutor raised no objection to a white juror who worked for the Post Office," Spital said. "Whatever one's views of capital punishment, it is both morally and constitutionally intolerable for a death sentence to be imposed if the defendant is innocent or if the verdict is marred by racial discrimination."
Son: 'He's going to be murdered'
The execution was to be carried out at the prison in Bonne Terre.
"Given the tenets of his religion (Islam) that he is very devout about, he believes it will be Allah's will and he is at peace with that," attorney Komp said before Greitens' decision. "Whatever will be will be. It's astounding."
Williams' son, Marcellus Williams II, said Monday that his father is not one to show pain.
"He's at peace. I think tomorrow he's going to be murdered. He (is) an innocent man, and that's not right," the younger Williams told CNN.
Williams II says he's never doubted his father's innocence and credited him with being a strong influence in his life.
"Someone murdered that woman, but it wasn't my father," he said. "I wish they would find the right suspect and charge them to the fullest extent of the law."
Gayle's widower, Dan Picus, is declining interviews, according to his wife.
In an editorial titled, "The death penalty debate hits close to home this time,"
the Post-Dispatch remembered Gayle and said she would not have favored the execution.
She "was a kind and gentle woman who went out of her way to do nice things for people. She'd left the newspaper in 1992 to do full-time volunteer work."
The editorial said the paper "opposes capital punishment under any and all circumstances, believing its administration is always arbitrary and always irrevocable. It has no deterrent value. If the state must execute, there must be no room for doubt."