"I can say with confidence there has never been a man or a woman more qualified than Hillary Clinton to serve as President of the United States of America," he declared.
Obama framed the choice facing voters in November as something bigger than "the usual debates between left and right."
"America is already great. America is already strong. And I promise you, our strength, our greatness, does not depend on Donald Trump," he said, questioning Trump's qualifications.
"He’s not really a plans guy. Not really a facts guy, either," he said of Trump. "He calls himself a business guy, which is true, but I have to say, I know plenty of businessmen and women who’ve achieved success without leaving a trail of lawsuits, and unpaid workers and people feeling like they got cheated."
Obama said Trump, whose signature line is "Make America Great Again," is "just offering slogans and he’s offering fear."
"He’s betting that if he scares enough people, he might score just enough votes to win this election," Obama said. "That is another bet that Donald Trump will lose. Because he’s selling the American people short. We are not a fragile or frightful people. Our power doesn’t come from some self-declared savior promising that he alone can restore order. We don’t look to be ruled."
"Anyone who threatens our values, whether fascists or communists or jihadists or homegrown demagogues, will always fail in the end," he added.
In a nod to the many Americans who believe the country is on the wrong track, Obama said they “still have more work to do," and said Hillary Clinton -- the "woman in the arena" -- would continue that work.
"There is only one candidate in this race who believes in that future, and has devoted her life to it; a mother and grandmother who’d do anything to help our children thrive ... the next President of the United States, Hillary Clinton," he said.
Clinton joined Obama on stage after his speech, embracing her former rival.
Trump campaign spokesman Stephen Miller panned the Democrats' night in a statement, saying they "offered no solution for the problems facing America."
Trump tweeted about Obama's speech today, saying the president "spoke last night about a world that doesn't exist."