The license, which was not announced publicly, was issued by Treasury's Office of Foreign Assets Control (OFAC), according to a January 15th letter to Gertler's lawyers that was obtained by The Sentry, a Washington D.C.-based anti-corruption group, and seen by Reuters.
It was not clear why the license was issued. A spokesman for Gertler welcomed the move but anti-graft campaigners urged President Joe Biden's Treasury to revoke the license.
A Treasury spokesman said the Biden administration was aware of the action, but declined to comment on whether it would move to undo it.
Treasury imposed the sanctions in December 2017 and June 2018, accusing Gertler of using his friendship with Democratic Republic of the Congo's former President Joseph Kabila to secure sweetheart mining deals worth more than a billion dollars.
The sanctions prohibited Gertler from doing business with US citizens, companies or banks, effectively barring him from doing transactions in dollars.
Gertler has always denied any wrongdoing and argued that his investments in Congo contributed significantly to the country's development.
While the license does not remove Gertler, an associate and more than 30 of his companies from the sanctions list, it authorizes, until January 31st, 2022, "all transactions and activities" otherwise prohibited by sanctions against them.
It also requires them to submit detailed reports to OFAC every 90 days about their financial activities.
"Mr. Gertler is grateful to OFAC for issuing a license that will enable him to operate his businesses and philanthropic activities," a spokesman said in a statement.
"He welcomes the reporting requirements and, with the oversight of a distinguished compliance team, looks forward to demonstrating that all of his activities fully comply with the letter and the spirit of the OFAC license and the law."
Brad Brooks-Rubin, The Sentry's managing director and a former Treasury official, said in a statement that the license served "no discernible geostrategic or national security purpose."
"Unlike the pardons and regulatory changes done openly, or any of Treasury's normal methods for undoing sanctions, this was done behind closed doors not only to the public but many professionals in the government as well," he said.
The Sentry called on Biden's Treasury Department to revoke the license.
One of the lawyers who lobbied OFAC on Gertler's behalf was Alan Dershowitz, who represented Trump in his first impeachment trial before the Senate last year, according to a disclosure form filed with Congress in 2018.Dershowitz did not immediately respond to an emailed request for comment.