"I will tell you at the time," said Trump, who has frequently discussed voter fraud and a "rigged" system.
"I'll keep you in suspense, OK?" Trump told debate moderator Chris Wallace.
Hillary Clinton responded, "That's horrifying."
Trump had just called the election "rigged."
"She's guilty of a very, very serious crime," he said of Clinton. "She should not be allowed to run. And just in that respect, I say it's rigged."
She said, "Every time Donald thinks things are not going in his direction, he claims whatever it is, is rigged against him ... There was even a time when he didn't get an Emmy for his TV program three years in a row and he started tweeting that the Emmys were rigged."
Trump responded, "Should have gotten it."
After the debate, Trump campaign manager Kellyanne Conway said of his refusing to say whether he will accept the results, "He said he's going to take a look at the results."
"Remember, Al Gore did concede," Conway told ABC News' Tom Llamas. "He conceded to Gov. George W. Bush and then called and rejected the concession and went on to contest the results. It went all the way to the Supreme Court. Election Day was early November, maybe Nov. 6 that year. And that case was decided on Dec. 12."
When Llamas asked Conway if she thought that was responsible for the democracy, she responded, "I think the most responsible thing that Donald Trump has done for democracy, frankly, was to run in the first place."
"And I credit him tremendously for making that sacrifice, he and his family. Because a lot of people have run in politics, Tom, for fame or fortune or status, prestige. He had all of that," she said. "And he sacrificed a great deal of that to do this. And the greatest gift he’s given to democracy is really to show people who have begged for years to have an outsider to disrupt the system to have somebody just come and turn the tables over completely. He was willing to do it."
Sean Spicer, the Republican National Committee's communications director, told ABC News after the debate that Trump will "accept the results of the election. One hundred percent."
"I think right now he's very concerned about the bias that exists in the media," Spicer said. "I think he wants to make sure people ... are focused on the election. But he will accept the results. No question about it."
"He's going to win this election soundly," Spicer said, "And this won't be an issue."
Clinton campaign chairman John Podesta said after the debate that Trump "made a tremendous mistake" by refusing to say whether he will accept the outcome.