President Biden issued a full and unconditional pardon of his son Hunter on Sunday night after repeatedly insisting he would not do so, using the power of his office to wave aside years of legal troubles, including a federal conviction for illegally buying a gun and for tax evasion.

In a statement issued by the White House, Mr. Biden said he had decided to issue the executive grant of clemency for his son “for those offenses against the United States which he has committed or may have committed or taken part in during the period from Jan. 1, 2014, through Dec. 1, 2024.”

He said he made the decision because the charges against Hunter were politically motivated and designed to hurt him politically.

“The charges in his cases came about only after several of my political opponents in Congress instigated them to attack me and oppose my election,” Mr. Biden said in the statement. “No reasonable person who looks at the facts of Hunter’s cases can reach any other conclusion than Hunter was singled out only because he is my son — and that is wrong.”

He added: “There has been an effort to break Hunter — who has been five and a half years sober, even in the face of unrelenting attacks and selective prosecution. In trying to break Hunter, they’ve tried to break me — and there’s no reason to believe it will stop here. Enough is enough.”

It was a remarkable turnaround for a man whose presidency and five-decade career was built in part on the idea that he would never interfere with the administration of justice. In 2020, he made the case that former President Donald J. Trump should be ousted from office to restore that kind of independence in America’s democracy, and he argued the same in 2024.

But in his statement, Mr. Biden sought to make the case for interfering after all, accusing his political enemies of going after his son in ways that anyone else would not have been. He said that he still believed in the justice system, but added, “I also believe raw politics has infected this process and it led to a miscarriage of justice — and once I made this decision this weekend, there was no sense in delaying it further.”

In fact, the president’s announcement came at the same time that Mr. Trump made it clearer than ever that his second term would be focused on retribution and revenge against Mr. Biden — with Hunter Biden as a prime target. The president-elect on Saturday said he would name Kash Patel, a loyalist who has vowed to go after Mr. Trump’s enemies, as F.B.I. director.

In his statement, Mr. Biden said, “I hope Americans will understand why a father and a president would come to this decision.”

After Mr. Biden announced the pardon, Hunter Biden issued a statement of his own.

“I have admitted and taken responsibility for my mistakes during the darkest days of my addiction — mistakes that have been exploited to publicly humiliate and shame me and my family for political sport,” he said. “I will never take the clemency I have been given today for granted and will devote the life I have rebuilt to helping those who are still sick and suffering.”

He expressed relief, but also some bitterness over what he perceived as an unnecessary prosecution, after his father told him he was being pardoned when the family gathered in Nantucket, Mass., for the Thanksgiving holiday, according to two people familiar with the situation.

Many of the president’s allies and critics had expected him to pardon his son, even though he and his spokeswoman had denied for months that he had any intention of doing so. NBC News first reported on Sunday evening that Mr. Biden had in fact decided to issue the pardon, which means his son will face no federal charges stemming from crimes he may have committed during that period.

But the move quickly drew expressions of scorn from Mr. Biden’s political adversaries.

In a post on social media, Mr. Trump called the pardon “Such an abuse and miscarriage of Justice!” He brought up the rioters from Jan. 6, 2021, some of whom he has suggested could be pardoned when he takes office.

Senator Charles E. Grassley, Republican of Iowa and a chief antagonist of Mr. Biden, said on social media that he was “shocked” that the president had pardoned his son because “he said many many times he wouldn’t & I believed him Shame on me.”

Jenna Ellis, a former lawyer for Mr. Trump’s 2020 election team, posted: “Joe Biden pardoned three turkeys this week,” a reference to the annual pardoning of two actual turkeys at the White House just before Thanksgiving.

The reversal by Mr. Biden came just 50 days before he is set to leave the White House and transfer power to Mr. Trump, who spent years attacking Hunter Biden over his legal and personal issues as a part of series of broadsides against the Biden family.

Mr. Biden for much of his time in office said he would refrain from commenting on high-profile criminal cases, even related to his son, to make good on a commitment to maintain the independence of the Justice Department.

After the president’s son was convicted on three federal felony counts for illegally buying a gun, Mr. Biden said he would not pardon or commute the sentence of his son.

“I said I’d abide by the jury decision,” Mr. Biden told reporters during the Group of 7 summit in June. “I will do that.”

The White House press secretary, Karine Jean-Pierre, repeatedly said that Mr. Biden would not issue a pardon for his son, often chiding reporters for asking the question.

In the summer of 2023, she was asked whether there was “any possibility” that the president would end up pardoning his son. She answered simply, “No.” When the reporter tried to ask the question again, she cut the question short and said: “I just said no. I just answered.”

​​Hunter Biden faced as much as 25 years in prison for lying on a federal form about his drug addiction when he bought a handgun in 2018, but he was unlikely to receive a sentence near that length. First-time offenders who did not use weapons for a violent crime typically receive much lighter sentences. Legal analysts had said it was possible that the president’s son could receive a year or less behind bars or even probation.

Justice Department officials have long been expecting — and dreading — the pardon of Hunter Biden, according to multiple law enforcement officials who spoke on condition of anonymity. Several law enforcement officials have for years described the case as a necessary but thankless task, given the political tempest around it and the intense personal dynamic between the president and his son.

It is not the first time a president has used his executive power to commute the sentence of a family member. On his last day in office, President Bill Clinton pardoned his half brother Roger Clinton for old cocaine charges. A month before leaving office, Mr. Trump pardoned his son-in-law Jared Kushner’s father, Charles Kushner, for tax evasion and other crimes.

Both Roger Clinton and Charles Kushner had long since completed their prison terms, and the pardons were about forgiveness or vindication rather than avoiding time behind bars. Over the weekend, Mr. Trump said that he would nominate Charles Kushner to be the U.S. ambassador to France.

Hunter Biden pleaded guilty in September to nine federal tax charges in Los Angeles after telling his legal team that he refused to subject his family to another round of anguish and humiliation after the gut-wrenching gun trial in Delaware earlier in the year.

The dramatic development signaled the final stages of a fraught investigation of more than five years into the period when Mr. Biden bankrolled his drug and alcohol addiction by leveraging his last name into lucrative overseas consulting contracts — and not paying taxes.

Mr. Biden had been set to remain free on bond until his sentencing hearing, which was scheduled for mid-December.