On Twitter last week, Assange's anti-secrecy site WikiLeaks posted, "If Obama grants Manning clemency Assange will agree to US extradition despite clear unconstitutionality of DoJ case."
Obama's move will test the promise. The president commuted Manning's 35-year sentence, freeing her in May, nearly three decades early. Manning has acknowledged leaking a trove of diplomatic cables and national security documents to WikiLeaks in 2010.
In a statement, Assange called Manning "a hero, whose bravery should be applauded."
Assange went on to demand that the U.S. government "should immediately end its war on whistleblowers and publishers, such as WikiLeaks and myself," but he made no mention of the Twitter pledge. His lawyer said he has been pressing the Justice Department for updates on an investigation concerning WikiLeaks.
Assange has been holed up for more than four years at the Ecuadorian Embassy in London. He has refused to meet prosecutors in Sweden, where he remains wanted on an allegation of rape, fearing he would be extradited to the U.S. to face espionage charges if he leaves the embassy.
The Justice Department has never announced any indictment of Assange, and it's not clear that any charges have been brought under seal.
The department, in refusing to turn over investigative documents sought by Manning under the Freedom of Information Act, has acknowledged that the FBI is continuing to investigate the publication of national security information on WikiLeaks arising from Manning's disclosures.
"That investigation concerns potential violations of federal criminal laws, in the form of serious threats to the national security, and the investigation continues today," Justice Department lawyers wrote in a court filing last year. "From the terms of her request, it is clear that Manning seeks to obtain documents concerning that investigation.
Separately, the FBI is also investigating Russian meddling through hacking in the U.S. presidential election. Hacked emails from top Democratic officials and Hillary Clinton campaign officials were posted on WikiLeaks in the final weeks of the presidential race.
With the commutation coming just days before Obama leaves office, any decision on whether to charge or seek to extradite Assange will now fall to the Trump administration.
In a statement Tuesday, a lawyer for Assange did not address whether Assange intended to come to the U.S.
"For many months, I have asked the DOJ to clarify Mr. Assange's status. I hope it will soon," Assange's lawyer, Barry Pollack, said in the statement. "The Department of Justice should not pursue any charges against Mr. Assange based on his publication of truthful information and should close its criminal investigation of him immediately."
Another Assange lawyer, Melinda Taylor, suggested he wouldn't go back on his word. "Everything that he has said he's standing by," she said in a brief telephone conversation with The Associated Press.
Associated Press writers Danika Kirka and Raphael Satter contributed from London.