Steve Bannon had refused to cooperate until now but his apparent change of heart is supported by Trump.
Former Trump strategist Steve Bannon has told the House select committee investigating the 6 January Capitol riots that he is willing to testify, according to US media.
Until now, he had refused to cooperate with the hearing.
In a letter seen by CBS News, his lawyer told the committee that Mr Bannon “is willing to, indeed prefers, to testify at your public hearing”.
Mr Trump said the testimony would be “in the best interests” of Americans.
The select committee has conducted a nearly year-long investigation into how Trump supporters invaded Congress on 6 January 2021 to disrupt lawmakers as they certified Democrat Joe Biden’s election victory.
Mr Bannon was summoned to testify on what he knew about the events leading up to the riot, but until now he had refused to comply.
He claims he was covered by “executive privilege”, a legal principle that protects many White House communications.
However, at the time of the Capitol riots, Mr Bannon was not an official advisor to the president, which the committee says means that executive privilege does not apply.
Mr Bannon’s testimony, if it goes ahead, could offer new insights into Trump’s inner circle in the hours leading up to and during the riots.
According to the committee, the former Trump strategist had specific knowledge about the events planned for 6 January before they took place.
Trump himself has become frustrated at the committee hearings, describing them as a “one-sided witch-hunt”, and has given his support for Mr Bannon’s testimony.
In a letter seen by the AP news agency, the former president said he was “waiving executive privilege for you, which allows for you to go in and testify truthfully and fairly, as per the request of the unselect committee of political thugs and hacks”.
However, the exact circumstances under which Mr Bannon will testify have not been agreed.
According to his lawyer Robert Costello, he wishes to appear before the committee in a public hearing, but standard procedure so far has been for testimony to be heard first in private interviews.
“We want to get all our questions answered, and you can’t do that in a live format,” committee member Zoe Lofgren – a Democrat – told CNN.
Mr Bannon is due to appear in court on 18 July on two charges of criminal contempt of Congress for previously refusing to testify and failing to submit documents.