US President Donald Trump has granted a rare posthumous pardon to legendary black boxer Jack Johnson.

Johnson, the first African-American to hold the world heavyweight title, from 1908 until 1915, was found guilty of sex trafficking in 1913 – a conviction which is widely seen as a racially charged legal travesty.

Johnson's crime was having crossed a state border with his white girlfriend, considered by the all-white jury to be in violation of the Mann Act, a federal anti-prostitution law which made it illegal to transport women across state lines for "immoral purposes" – although the law had not actually passed at the time of the alleged offense.

"I am taking this very righteous step, I believe, to correct a wrong that occurred in our history and to honor a truly legendary boxing champion," said President Trump on Thursday at a ceremony in the Oval Office.

After his conviction, Johnson initially fled the country while on bail, first to Canada, then to France, living as a fugitive for seven years before returning to the United State in 1920 and handing himself in. He then served 10 months in jail for what Trump described as "what many view as a racially-motivated injustice."

Trump said the case was first brought to his attention by "Rocky" star Sylvester Stallone, whom he credited with championing the pardon.

Stallone attended the Oval Office ceremony along with retired heavyweight title holder Lennox Lewis, WBC heavyweight champion Deontay Wilder and other dignitaries from the world of boxing.

Linda Haywood, a great-great-niece of Johnson who also attended, thanked Trump for "rewriting history" after former presidents George W Bush and Barack Obama had failed to act on previous calls for a pardon.

Johnson died in a car accident in 1946, aged 68, but was later the subject of the 2005 documentary "Unforgiveable Blackness."

mf/pfd (dpa/AP)