President Joe Biden has pardoned all Americans who have been convicted at the national level of possessing small amounts of marijuana.
Officials estimate about 6,500 people with federal convictions for simple possession of marijuana will benefit.
No-one is currently in federal prison solely for possession of marijuana. Most convictions occur at state level.
But the federal pardons will make it easier for people to get employment, housing, and education, Mr Biden said.
As a presidential candidate, Mr Biden promised to decriminalise cannabis use, as well as expunging convictions.
"Sending people to prison for possessing marijuana has upended too many lives and incarcerated people for conduct that many states no longer prohibit," Mr Biden said on Thursday.
He added that non-white people were statistically far more likely to be jailed for cannabis.
As a White House candidate, Mr Biden was criticised for writing a 1994 crime bill that stiffened penalties for drug crimes and led to more incarceration of minorities.
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The Democratic president said he would call upon all state governors to issue their own marijuana pardons.
He is also directing the Department of Justice and the Department of Health to review how cannabis is classified under federal law.
"We classify marijuana at the same level as heroin – and more serious than fentanyl," said Mr Biden. "It makes no sense."
Recreational marijuana is already legal in 19 states and Washington DC. Medical use is legal in 37 states and three US territories.
However, the drug remains illegal at the federal level, even in states where it can be legally bought and used, meaning people there could still be convicted for possession in certain circumstances.
The pardons come a month before November's congressional mid-term elections, which will determine the power balance in Washington for the last two years of Mr Biden's term.
Life for Pot, a website advocating for the release of non-violent marijuana offenders, noted that there are no known federal prisoners that will be affected by Mr Biden's measure, tweeting: "This is window dressing."
Cannabis company shares jumped on the stock market by around 20% with news of Mr Biden's pardons.
Mr Biden is not the first US president to pardon cannabis offenders.
On his final day in office, Donald Trump pardoned 12 marijuana offenders, including some who had been jailed for life under the three-strikes rule created by Mr Biden's 1994 crime bill.
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