Counterintelligence investigator Peter Strzok was removed from special counsel Robert Mueller’s Russia probe this year, after Justice Department investigators began reviewing whether investigators had shown bias against Mr Trump.
Mr Strzok's reference to Trump as an "idiot" came in an email exchange with another agent, Lisa Page, in August 2015.
“I just saw my first Bernie Sanders bumper sticker. Made me want to key the car,” Ms Page wrote.
“He’s an idiot like Trump. Figure they cancel each other out,” Mr Strzok replied.
In a March 2016 message, Ms Page exclaimed: "God Trump is a loathsome human… OMG he’s an idiot.”
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“He's awful,” Mr Strzok replied.
Special counsel Robert Mueller removed Mr Strzok from his Russia election meddling probe this summer after becoming aware of the emails.
Ms Page by that time had already ended her assignment in Mr Mueller's office, according to Politico, one of several US news organisations that obtained copies of the email.
The messages turned up as part of a Justice Department inspectors’ general investigation into potential political influence on investigative decisions during the campaign.
Some Republicans have urged that a second special counsel be named to investigate Mr Mueller's operation. At least 375 texts were turned over by the Justice Department.
Mr Mueller is looking into whether Trump campaign aides or associates colluded with Russian efforts to sway the 2016 election in the Republican real estate mogul's favour.
Former Trump national security advisor Michael Flynn and George Papadopoulos, a campaign foreign policy advisor who maintained Russian contacts, have pleaded guilty to lying to the FBI.
Meanwhile, former Trump campaign Paul Manafort and his partner Rick Gates have pleaded not guilty to money-laundering charges related to their previous political work in Ukraine.
