He added that he would accept a “clear” result – but reserved the right to challenge a “questionable” result.
He appeared at a rally in Delaware, Ohio, speaking for the first time since the third televised debate with Democratic opponent Hillary Clinton.
Mr Trump has been heavily criticised for suggesting that he might not accept the election result.
Polls suggest Mrs Clinton is ahead nationally and in key battleground states.
Speaking in Ohio, Mr Trump said, grinning: “I would like to promise and pledge to all of my voters and supporters and to all of the people of the United States, that I will totally accept the results of this great and historic presidential election – if I win.”