PHILADELPHIA - Pennsylvania’s highest court threw out Bill Cosby’s sexual assault conviction and released him from prison Wednesday in a stunning reversal of fortune for the comedian once known as “America’s Dad.”

The court ruled that the prosecutor who brought the case was bound by his predecessor’s agreement not to charge Mr. Cosby. Mr. Cosby, 83, flashed the V-for-victory sign to a helicopter overhead as he trudged into his suburban Philadelphia home after serving nearly three years of a three- to 10-year sentence for drugging and violating Temple University sports administrator Andrea Constand in 2004.

The former “Cosby Show” star — the first celebrity tried and convicted in the #MeToo era — had no immediate comment. “The Pennsylvania high court got it right,” said veteran Richmond defense attorney David P. Baugh. “This is not about guilt or innocence. You just cannot use testimony that a defendant provides after being granted immunity to convict a defendant. I have won cases on that very point.”

Now in his 46th year as a practicing lawyer, Mr. Baugh said that the initial prosecutor “might have been excessive” in providing such a sweeping immunity deal to Mr. Cosby in exchange for disclosing his sexual misconduct, but once that happened, the “second prosecutor had no business trying to bring a case. The case was dead at that point or should have been.”

Mr. Baugh said the lower court judge is most responsible for allowing the case to go forward when the immunity deal was known. But he said that just because people put on black robes does not mean they know anything about the law. He noted a longtime legal joke involves the question of what to call a lawyer with an I.Q. of 50. The joke’s answer: “ ‘Your honor.’ That judge had the legal I.Q. of an orange,” Mr. Baugh said.

Mr. Cosby was arrested in 2015, when a district attorney armed with newly unsealed evidence — the comic’s damaging deposition in a lawsuit brought by Ms. Constand — filed charges against him just days before the 12-year statute of limitations was about to run out.

But the Pennsylvania Supreme Court said Wednesday that District Attorney Kevin Steele, who made the decision to arrest Mr. Cosby, was obligated to stand by his predecessor’s promise not to charge Mr. Cosby, though there was no evidence that agreement was ever put in writing.

Justice David Wecht, writing for the majority in the 4-3 ruling, said Mr. Cosby had relied on the previous district attorney’s decision not to charge him when the comedian gave his potentially incriminating testimony in Ms. Constand’s civil case.

The court called Mr. Cosby’s subsequent arrest “an affront to fundamental fairness, particularly when it results in a criminal prosecution that was forgone for more than a decade.” It said justice and “fair play and decency” require that the district attorney’s office stand by the decision of the previous prosecutor.

The justices said that overturning the conviction and barring any further prosecution “is the only remedy that comports with society’s reasonable expectations of its elected prosecutors and our criminal justice system.”

As Mr. Cosby was promptly set free from the state prison in suburban Montgomery County, Penn., and driven home, his appeals lawyer, Jennifer Bonjean, said he should never have been charged.

“District attorneys can’t change it up simply because of their political motivation,” she said, adding that Mr. Cosby remains in excellent health, apart from being legally blind.

In a statement, Mr. Steele said Mr. Cosby went free “on a procedural issue that is irrelevant to the facts of the crime.” He commended Ms. Constand for coming forward and added, “My hope is that this decision will not dampen the reporting of sexual assaults by victims.”

Ms. Constand and her lawyer did not immediately return messages seeking comment.

“FINALLY!!!! A terrible wrong is being righted — a miscarriage of justice is corrected!” “Cosby Show” co-star Phylicia Rashad tweeted.

“I am furious to hear this news,” actor Amber Tamblyn, a founder of Time’s Up, an advocacy group for victims of sexual assault, said on Twitter. “I personally know women who this man drugged and raped while unconscious. Shame on the court and this decision.”

Peter Goldberger, a suburban Philadelphia lawyer with an expertise in criminal appeals, said prosecutors could ask the Pennsylvania Supreme Court for re-argument or reconsideration, but it would be a very long shot.

“I can’t imagine that with such a lengthy opinion, with a thoughtful concurring opinion and a thoughtful dissenting opinion, that you could honestly say they made a simple mistake that would change their minds if they point it out to them,” Mr. Goldberger said.

Even though Mr. Cosby was charged only with the assault on Ms. Constand, the judge at his trial allowed five other accusers to testify that they, too, were similarly victimized by Mr. Cosby in the 1980s. Prosecutors called them as witnesses to establish a pattern of behavior on Mr. Cosby’s part.

Mr. Cosby’s lawyers had argued on appeal that the use of the five additional accusers was improper. But the Pennsylvania high court did not weigh in on the question, saying it was moot, given the finding that Mr. Cosby should not have been prosecuted in the first place.

In New York, the judge at last year’s trial of Hollywood mogul Harvey Weinstein, whose case helped sparked the #MeToo movement in 2017, let four other accusers testify. Mr. Weinstein was convicted and sentenced to 23 years in prison.

In sentencing Mr. Cosby, the trial judge had ruled him a sexually violent predator who could not be safely allowed out in public and needed to report to authorities for the rest of his life.

In May, Mr. Cosby was denied parole after refusing to participate in sex offender programs behind bars. He said he would resist the treatment programs and refuse to acknowledge wrongdoing even if it meant serving the full 10 years.

The groundbreaking Black actor grew up in public housing in Philadelphia and made a fortune estimated at $400 million during his 50 years in the entertainment industry that included the TV shows “I Spy,” “The Cosby Show” and “Fat Albert,” along with comedy albums and a multitude of television commercials.

The suburban Philadelphia prosecutor who originally looked into Ms. Constand’s allegations, Montgomery County District Attorney Bruce Castor, considered the case flawed because Ms. Constand waited a year to come forward and stayed in contact with Mr. Cosby afterward. Mr. Castor declined to prosecute and instead encouraged Ms. Constand to sue for damages.

Questioned under oath as part of that lawsuit, Mr. Cosby said he used to offer Quaaludes to women he wanted to have sex with. He eventually settled with Ms. Constand for $3.4 million.

Portions of the deposition later became public at the request of The Associated Press and spelled Mr. Cosby’s downfall, opening the floodgates on accusations from other women and destroying the comic’s good guy reputation and career. More than 60 women came forward to say Mr. Cosby violated them.

The AP does not typically identify sexual assault victims without their permission, which Ms. Constand has granted.

Mr. Cosby, in the deposition, acknowledged giving Quaaludes to a 19-year-old woman before having sex with her at a Las Vegas hotel in 1976. Mr. Cosby called the encounter consensual.

On Wednesday, the woman, Therese Serignese, now 64, said the court ruling “takes my breath away.”

“I just think it’s a miscarriage of justice. This is about procedure. It’s not about the truth of the women,” Ms. Serignese said, adding she took solace in the fact that Mr. Cosby served nearly three years behind bars. “That’s as good as it gets in America” for sex crime victims, she said.