Obama has assailed his successor as deeply unfit for the office he occupies and argued that voting for his former No.2, Joe Biden, was necessary to ensure the very survival of American democracy.
Obama has mostly avoided directly criticising Trump over the last four years, and his speech on night three of the Democratic National Convention (DNC), marked an unusually harsh assessment of one president by another.
Barack Obama's speech at the 2020 Democratic National Convention. Post continues below video.
His assertion that Trump, a Republican, is incapable of meeting the demands of the presidency echoed the remarks from his wife, Michelle Obama on Monday, that Trump "simply cannot be who we need him to be".
Biden was formally nominated this week to take on Trump in the November 3 presidential election, with US senator Kamala Harris his choice for vice-president.
Barack Obama's speech.
Before he even opened his mouth, Obama's chosen location for his speech spoke volumes. He was in Philadelphia, at a museum about the writing of the American constitution.
The speech, during the third night of the Democratic National Convention, had a sense of urgency as Obama explained how crucial it was for Americans to vote Trump out of office in November for the sake of its democracy and everyone in it.
He said Trump was simply incapable of being president.
"I did hope, for the sake of our country, that Donald Trump might show some interest in taking the job seriously.
"That he might come to feel the weight of the office. And discover some reverence for the democracy that had been placed in his care. But he never did.
"For close to four years now, he's shown no interest in putting in the work; no interest in finding common ground; no interest in using the awesome power of his office to help anyone but himself and his friends; no interest in treating the presidency as anything but one more reality show that he can use to get the attention he craves."