WASHINGTON — President Trump on Thursday pardoned Jack Johnson, the first black heavyweight champion, who was convicted in 1913 of transporting a white woman across state lines. Mr. Trump signed the pardon for Johnson during an Oval Office ceremony, sitting at the Resolute Desk and flanked by Sylvester Stallone, Lennox Lewis and other fighters.

The president called Johnson “a truly great fighter, had a tough life,” but served 10 months in federal prison “for what many view as a racially-motivated injustice.”

Mr. Trump said the conviction took place during a “period of tremendous racial tension in the United States.”

Decades after Johnson was convicted under the Mann Act, his case drew significant attention as a gross miscarriage of justice and a symbol of the depths of racism in the American justice system.

Johnson was convicted in 1913 of violating the Mann Act, also known as the White-Slave Traffic Act, on charges that he transported a white woman across state lines “for immoral purposes.” The woman Johnson transported, Belle Schreiber, worked as a prostitute and had been in a steady relationship with the heavyweight champion.

Johnson was sentenced to a year in prison, but he fled the country for several years, returning in 1920 to serve his sentence.

“He was treated very rough, very tough,” Mr. Trump said Thursday as he signed what he called an “executive grant of clemency, a full pardon” to Johnson.

The president noted that bipartisan requests for the pardon for Johnson had been made for years, but that despite that, no previous president had been willing to sign one.  He noted — with a glancing reference to former President Barack Obama — that the last resolution in Congress calling for the pardon was in 2015.

“They couldn’t get Obama to sign it,” Mr. Trump said.

The World Boxing Council, one of boxing’s sanctioning bodies, invited the current and former champions, including the American Deontay Wilder and Lewis of Britain, to the ceremony, according to Tim Smith, the vice president of communications for Haymon Boxing.

The son of former slaves, Jack Johnson was a powerhouse boxer who began boxing professionally in the late 1890s and continued to do so until 1915.

In his career, Johnson had 73 professional wins, including 40 that ended via knockout.  “He is one of the craftiest, cunningest boxers that ever stepped into the ring,” legendary boxer John L. Sullivan once said about Johnson.

Not only was Johnson the first black man to win the heavyweight world championship, but he also was the rare black man of his era who was brash, ostentatious and unapologetic about his wealth and success. He taunted his opponents in the ring and dated white women, which was taboo at the time.

Johnson’s persona and race led to harsh coverage from newspapers over the years, which only served to further a negative image of the fighter.

“Jack Johnson lived in the lap of luxury, abused the fame and fortune that came to him, and died bereft of riches,” read an Associated Press article that ran in The New York Times after he died in 1946.

But in the decades after Johnson died, as society became more enlightened, his conviction came to be seen as a miscarriage of justice. Politicians and celebrities including John McCain, Stallone and the filmmaker Ken Burns advocated for his pardon.

The Obama administration passed on pardoning Johnson, citing in part allegations of domestic violence against women. But last month, President Trump tweeted that he was considering pardoning Johnson after Stallone had told him about the boxer’s story.

Johnson’s 1910 fight against James J. Jeffries inspired the 1967 play and 1970 movie “The Great White Hope.”

After Johnson had won the heavyweight title in 1908, many in white society advocated for a white fighter to step up and win the title back.

Jeffries, a former champion who had been in retirement, took up that challenge. But Johnson decimated Jeffries, a victory that sparked violent white backlash in the form of riots across the country.