The threat appeared to be connected to a far-right conspiracy theory, mainly promoted by supporters of QAnon, that Mr Trump will rise again to power on March 4.
That was the original presidential inauguration day until 1933, when it was moved to January 20.
The revelation was detailed in a statement from the Capitol Police. It came at the same time as the acting police chief testifying before a House subcommittee.
“The United States Capitol Police Department is aware of and prepared for any potential threats towards members of Congress or towards the Capitol complex,” the agency said in a statement.
“We have obtained intelligence that shows a possible plot to breach the Capitol by an identified militia group on Thursday March 4.”
The statement differs from an advisory that was sent to members of Congress by the acting House sergeant-at-arms this week.
That said Capitol Police had “no indication that groups will travel to Washington DC to protest or commit acts of violence”.
Police did not identify the militia group in the statement on Wednesday.
In her testimony to the House panel, acting Capitol Police chief Yogananda Pittman said her investigators had collected “some concerning intelligence”.
But Pittman declined to provide any details publicly, saying it was “law enforcement sensitive” and that she would provide a private briefing for the subcommittee members.
Capitol Police said they have stepped up security around the complex since January’s insurrection.
They added physical security measures such as the fencing topped with razor wire around the Capitol and members of the National Guard, who remain at the complex.
The statement said the agency was “taking the intelligence seriously” but provided no other specific details on the threat.
The announcement comes as the Capitol police and other law enforcement agencies are taking heat from Congress in contentious hearings this week on their handling of the January 6 riot.
They were prepared for a protest and were badly underprepared for the riot.
It took hours for reinforcements to come and by then Trump supporters had roamed the halls of the US Capitol for hours.
March 4 is considered by some to be the “real inauguration day”, although there has not been nearly the amount of online chatter that occurred before January 6 from extremist groups.
So far, about 300 people have been charged with federal crimes for their roles in the riot.
Five people, including a Capitol Police officer, died.
Thousands of accounts that promoted the January 6 event that led to the violent storming of the Capitol have since been suspended by major tech companies like Facebook and Twitter, making it far more difficult for QAnon and far-right groups to organise a repeat of the mass gathering on Thursday.