Wolf's decision means there will be no execution on March 4 for Terrance Williams, 48, a former star quarterback at Germantown High School sentenced to death for the 1984 murder of Amos Norwood, a 56-year-old Germantown church volunteer.
Wolf's predecessor, Gov. Tom Corbett, signed Williams' death warrant last month.
In reversing that decision and announcing the moratorium, the new governor said it "is in no way an expression of sympathy for the guilty on death row, all of whom have been convicted of committing heinous crimes, and all of whom must be held to account.
"The guilty deserve no compassion, and receive none from me," Wolf added. "I have nothing but the deepest appreciation for the work of victim advocates, and sympathize and stand with all those who have suffered at the hands of those in our society who turn to violence."
Williams' lawyers have been fighting for years to stop his execution, contending, among other reasons, that prosecutors withheld evidence that Williams' victim had sexually abused teenage boys. After the reprieve, Shawn Nolan, chief of the Federal Defender's death penalty unit in Philadelphia, thanked Wolf on Williams' behalf.
"The Pennsylvania Senate has recognized the need for the commonwealth to examine the capital punishment system," Nolan said. "In light of the ongoing bipartisan state legislative commission, and given all of the well-documented problems with Pennsylvania's death penalty, the governor's decision to grant a reprieve and issue a moratorium is appropriate."
In 2011, the state Senate authorized a task force to comprehensively study the death penalty and whether it can be legally and effectively administered in Pennsylvania. The task force was to have issued its report in December 2013, but the deadline has been extended.
Thirty-two states have capital punishment, including Pennsylvania and Delaware. New Jersey abolished the death penalty in 2007.
In Pennsylvania, the debate about capital punishment - why it is not used more often versus why it should be allowed at all - has never stopped.
Wolf's order ignited it anew, as well as questions about his power to do so.
House Speaker Mike Turzai (R., Allegheny) and Majority Leader Dave Reed (R., Indiana) accused Wolf of "overstepping his authority."
"His death penalty moratorium is, in reality, a political statement without public discourse or input. Or, apparently, without any consideration for those the victims left behind," the GOP leaders said in a statement.
Senate President Pro Tempore Joe Scarnati (R., Jefferson) and Senate Majority Leader Jake Corman (R., Centre) agreed, calling the moratorium "ideologically driven."
But State Sen. Daylin Leach (D., Montgomery) used the occasion to reintroduce his bill - for the fourth consecutive session - to abolish Pennsylvania's death penalty.
Outside the Capitol, the Pennsylvania District Attorneys Association called Wolf's action "a misuse of his power [that] ignores the law" and said he had "turned his back on the silenced victims of cold-blooded killers.
"A moratorium is just a ploy," the prosecutors association added. "Make no mistake, this action is not about waiting for a study - it's about the governor ignoring duly enacted law and imposing his personal views against the death penalty."
Philadelphia District Attorney Seth Williams called Wolf's action "an injustice to the citizens of this state."
"If the governor wants to be a man of his convictions, he should debate this issue publicly and try to persuade the legislature and the people to change the law," Williams said. "But he has no moral or legal right to nullify judicial rulings and legislative statutes."
Williams said the people most grateful for Wolf's moratorium are "the guiltiest, cruelest, most vicious killers on death row."
The Pennsylvania State Troopers Association called the moratorium a "travesty" and "a sad day for Pennsylvania."
It cited the moratorium's potential impact on prosecutors' announced intent to seek the death penalty for Eric Frein, charged with the Sept. 12 ambush killing of State Police Cpl. Bryon Dickson and wounding of Trooper Alex Douglass in Northeast Pennsylvania.
Marc Bookman, director of the Philadelphia-based nonprofit Atlantic Center for Capital Representation, said the governor's action was "very thoughtful considering the flaws that seem more apparent every day."
Philadelphia Catholic Archbishop Charles Chaput said he was "very grateful to Gov. Wolf for choosing to take a deeper look into these studies, and I pray we can find a better way to punish those who are guilty of these crimes."
Pennsylvanians for Alternatives to the Death Penalty released statements from six members, including relatives of murder victims such as Megan Smith, whose father and stepmother were murdered in 2001 in Lancaster County.
"I applaud Gov. Wolf for recognizing that Pennsylvania's capital punishment system is broken in so many ways," Smith said. "It costs far more than imprisoning murderers for life. It is inconsistent and arbitrary, and it sometimes sentences innocent people to die."
Pennsylvania authorizes the death penalty only for people convicted of first-degree murder: a malicious premeditated killing. If the prosecutor does not seek the death penalty in a first-degree murder case, or if a jury does not impose a death sentence after a defendant is found guilty of first-degree murder, the sentence is life in prison without chance of parole.
In the abstract, Pennsylvanians overwhelmingly support the death penalty. A 2003 poll by Quinnipiac University showed that 67 percent favored the death penalty, and 28 percent opposed it, and 4 percent responded "Don't know."
When the same poll asked respondents to choose between the death penalty and life in prison with no chance of parole, support for capital punishment fell to 50 percent. Forty-two percent favored life in prison, and 8 percent responded, "Don't know."
Regardless of the polls, Pennsylvania juries have shown increasing reluctance to impose a death sentence when it is an option. Only three people were sentenced to death in 2014, from Fayette, York, and Montgomery Counties, and only four in 2013.
Of the three people executed since Pennsylvania reinstated capital punishment in 1978 - two in 1995, the last in 1999 - all three had ended their appeals and asked to be executed.
As of Feb. 2, according to state prison officials, 183 men and three women were on death row in Pennsylvania, confined to their cells 23 hours a day.
Wolf during the campaign said he would not sign death warrants until concerns have been addressed about avoiding executing an innocent person.
"Our concern is about what to do to ensure that in executing people we are as certain as certain can be that we are not executing people who are innocent," said Marissa Bluestine, legal director of the Pennsylvania Innocence Project.
Philadelphia civil rights lawyer David Rudovsky noted that - beyond the question of innocence - more than 100 Pennsylvanians have been taken off death row because their trials were found to have been unfair.
During his term as governor, Corbett signed 48 death warrants, although no executions occurred under his watch.
Last fall, Corbett stayed the execution of Hubert L. Michael Jr., who confessed to murdering a York County teenager two decades ago. Corbett's decision came after state officials could not acquire lethal injection drugs.
The drugs needed for lethal injections are becoming harder for states to obtain because some manufacturers have refused to sell them for that purpose. Some states, including Pennsylvania, have resorted to obtaining them from compounding pharmacies.
Pennsylvania's death penalty is also under at least two challenges in the state and federal courts.
On Friday, U.S. District Judge Yvette Kane in Harrisburg granted judgment to the commonwealth in a suit challenging the death penalty for violating the U.S. Constitution's Eighth Amendment ban on cruel and unusual punishment.
Rudovsky, who filed the suit, said he was unsure if he would appeal Kane's ruling to the U.S. Court of Appeals for the Third Circuit.
Rudovsky said he would go before the state Commonwealth Court on March 11 in a related challenge, arguing that the state cannot legally execute anyone using other than the original drug cocktail the legislature wrote into the 1978 law reinstating capital punishment.
Rudovsky said the state law requires use of sodium thiopental, a "ultra short-acting barbiturate," to anesthetize the condemned person before the lethal drugs are administered. The drug is no longer manufactured because of its use in capital punishment.
Inquirer staff writer Amy Worden contributed to this article.