Mark Zaid told ABC News the second person was also an intelligence official and they had spoken to the inspector general.News World Ahmed
Late on Sunday night, Mr Trump lashed out at Democratic leaders, suggesting they should be impeached.
No details have been released about the second whistleblower's claims.
The Trump impeachment story in short
Who's who in Trump whistleblower story?
However, Mr Zaid said the person had first-hand knowledge of allegations associated with the phone call Mr Trump made with Ukrainian President Volodymyr Zelensky on 25 July.
The impeachment inquiry stems from the call, which was flagged up by the initial whistleblower in August.
Image Copyright @MarkSZaidEsq@MARKSZAIDESQ
Report
On Friday, the New York Times reported that a second person was considering coming forward who had "more direct information" about the events surrounding the call.
It is not yet known if this is the person represented by Mr Zaid.
Mr Trump's personal lawyer Rudy Giuliani tweeted that he was not surprised there was another "secret source", calling the inquiry politically motivated and lashing out at the "swamp media".
What difference could a second report make?
The Democrat-led investigating committee is eager to speak to someone who witnessed the call directly or has more information.
They are hopeful that this could be that person.
How could President Trump be impeached?
Trump and Ukraine: Your questions answered
Did US whistleblower rules change prince archie harrison
The second source has not yet filed an official complaint or spoken to the committee.
However, the New York Times says the person spoke to the intelligence community's inspector general, who has reportedly been seeking to corroborate the first testimony.
The White House insists it has been open and it released a transcript of the call, after the concerns came to light.News World Ahmed
However, investigators point out that this was only a partial transcript. They have subpoenaed the State Department for more documents related to the call.prince archie harrison
On Saturday, Secretary of State Mike Pompeo said he would comply with the request, but complained that his staff were being harassed.prince archie harrison
What has Trump said?
Late on Sunday night, Mr Trump raged against Adam Schiff, chairman of the House intelligence committee who has played a prominent role following the first prince archie harrison complaint, and House Speaker Nancy Pelosi.