"Chelsea Manning is somebody who went through the military criminal justice process, was exposed to due process, was found guilty, was sentenced for her crimes, and she acknowledged wrongdoing", he said.
President Obama commuted Chelsea Manning's prison sentence on January 17.
Assange, now living in the Ecuadorian embassy in London, is subject to an extradition order to Sweden and says he fears extradition to the U.S. for his role in publishing leaked, classified information.
The former army intelligence analyst, who leaked thousands of classified documents to Wikileaks in 2010, will now be released on May 17.
Manning, a transgender woman who was born Bradley Edward Manning, was convicted in 2013 for leaking three-quarters of a million classified or sensitive military and diplomatic documents that were published by WikiLeaks.
Mr Assange, who has been holed up at Ecuador's London embassy since 2012, reiterated his pledge in an online news conference today.
He was interviewed in the embassy in November in the presence of prosecutors from Sweden, where he faces a sex allegation.
Although Julian Assange has promised that he will hand himself to the US authorities in exchange for Chelsea Manning's freedom - he has refused to do so.
The WikiLeaks founder also said a return to the United States is possible as long as the U.S. government ensures his are protected.
She was handed the longest ever sentence for leaking information - 35 years - and would have stayed in prison until 2045.
"Julian Assange said "a 14 year old could have hacked Podesta" - why was DNC so careless?"
However, the DOJ on Wednesday declined to comment on the matter and wouldn't say whether the US government is even seeking Assange's extradition from the United Kingdom.
He said if it took him going to the United States to "flush out" the case being prepared against him, or to drop it, then "we are looking at that".