Tom Coburn, who served the US state of Oklahoma in the House of Representatives and then the Senate from 1995 to 2015, made the comment in an interview with the New York Times published on Thursday.
"We have a leader who has a personality disorder," Coburn said, without elaborating on his medical opinion.
The former Republican senator, however, said that the party is not going to stop supporting the billionaire businessman, because "[Trump's] done what he actually told the people he was going to do, and they're not going to abandon him."
Coburn’s remarks came fresh after a number of senior Republicans rebuked the US president, with Senator Jeff Flake launching a blistering attack on Trump, saying that his behavior was "dangerous to democracy.”
"Reckless, outrageous and undignified behavior has become excused and countenanced as telling it like it is when it is actually just reckless, outrageous and undignified," Flake said. "And when such behavior emanates from the top of our government, it is something else. It is dangerous to a democracy."
The Arizona lawmaker also announced he would not seek re-election in 2018, arguing that he had “children and grandchildren to answer to."
Republican Senator Bob Corker said on Wednesday, "He’s obviously not going to rise to the occasion as President," and that, “the debasement of our nation” would be what Trump “will be remembered most for.”
"I’ve been with him on multiple occasions to create some kind of aspirational approach, if you will, to the way he conducts himself. But I don’t think that that’s possible. Unfortunately, I think world leaders are very aware that much of what he says is untrue,” he added.
Trump is often alleged to have a "narcissistic personality disorder" or some other psychological conditions.
Earlier this month, an organization of psychologists, Duty to Warn PAC, held marches across the US to tell the public that the businessman-turned politician suffers from malignant narcissism.
Professionals claim Trump is also displaying warning signs such as scapegoating and banishing those who are seen as threats; degrading, ridiculing, and demeaning rivals and critics; as well as fostering a cult of “the strong man” who appeals to fear and anger. They also criticize the US president for showing little concern for truth and feeling no need for rational persuasion.
During the 2016 presidential race, his Democratic rival, Hillary Clinton, famously claimed that Trump was "temperamentally unfit" for the job.
A recent Fox News opinion poll shows Trump's approval rating has dipped to the lowest mark it has been in any of the surveys conducted by the news channel.
Only 38 percent of voters approved the president’s performance, which is a 4 percentage points drop from September and a full 10 points since the poll's first post-inaugural results in February, according to the survey.
The poll also found that 57 percent of voters disapproved the president’s job performance.