Trump on Saturday shared a post from a user named @surfermom77, who described themselves as a "100% Trump supporter", the BBC reported.
The retweet was later removed from the President's Twitter timeline but could still be found via a direct link.
In response to his retweet on Saturday, attorney Stephen Kohn, an expert in whistleblower protection laws, told the Washington Post that the President was violating his duty to safeguard whistleblowers.
"The paradox is that it was the president's duty to protect this person," the BBC quoted Kohn as saying to the Post.
"It's inconceivable that he not only doesn't do it, but violates it."
Michael German, who left the FBI after reporting allegations of mismanagement, said it was "completely inappropriate for the President of the US to be engaged in any type of behaviour that could harm a whistleblower".
In August, the whistleblower filed a report expressing concern over a phone call on July 25 in which Trump asked his Ukrainian counterpart Volodymyr Zelensky to investigate Joe Biden, former Vice President and a Democratic front-runner for the 2020 US presidential election.
The whistleblower's report ultimately led to an inquiry in the Democrat-controlled House of Representatives which culminated in Trump's impeachment on December 18 on charges of abuse of power and obstruction of Congress, making him the third President in US history to be impeached.
Trump now faces a trial in the Republican-controlled Senate, which is expected to acquit him along party lines.
( With inputs from IANS )