The online survey found 47 percent of likely voters saying they would vote for Obama and 45 percent for Romney if the Nov. 6 election were held now. That solidifies an improvement by the Republican who had trailed his opponent by six points in the same daily poll going into the debate.
"Romney's performance in the debate I think has improved his share of the vote for now ... It's a significant change from where we were a couple of weeks ago," said Ipsos pollster Julia Clark.
But the upside for Romney from the debate, the first of three with Obama this month, appears limited.
"I would say that if the debate was a game-changer, we would see Romney continue to make gains," she said. "He's narrowed the race but he doesn't seem to be overtaking Obama."
The division among likely voters was exactly the same in the rolling poll on Saturday and 46 percent to 44 percent on Friday.
Fifty-five percent of registered voters thought Romney did better at the debate, where he was aggressive in attacking the White House's economic record. Obama's muted performance at the podium received approval from less than 25 percent.
The pool of voters Obama and Romney are fighting for is narrowing.
In Sunday's poll, 8 percent of registered voters said they have already voted early in person or by absentee ballot, while 84 percent said they have "definitely" decided which candidate to vote for, leaving only 16 percent saying they may change their mind. And even fewer will actually do so, Clark said.
In a positive move for Obama's campaign, however, Friday brought a surprisingly strong report on U.S. employment. On Saturday, the campaign announced raising $181 million in September - a record so far for the 2012 election.
But Clark said the polls are unlikely to reflect the more positive jobs report.
"Americans don't change their views on how things are doing economically based on jobs numbers," but focus on their personal experience, she said.
This week, the focus of the campaign shifts to the debate on Thursday between Vice President Joe Biden and the Republican nominee to replace him, Wisconsin Congressman Paul Ryan.
The precision of Reuters/Ipsos polls is measured using a credibility interval. In this case, the poll has a credibility interval of plus or minus 2.7 percentage points for registered voters and 2.9 points for likely voters. It was conducted Oct. 3-7.
The poll interviewed 1,745 registered voters and 1,490 likely voters over the previous four days.