Obama leads Romney 48 percent to 44 percent, Gallup said late Tuesday. It is the sixth consecutive day in which the poll has showed the president with a small, but statistically significant lead.
The margin of error is 2 percentage points.
The shift comes after almost two months in which the two candidates were essentially tied in the poll's results, Gallup said.
A rolling average of seven major polls by the RealClearPolitics political news and polling data aggregator shows Obama with a slim 2.6 percentage-point average lead Wednesday, a United Press International review indicated.
The polls included in the average are Gallup Daily Tracking, Rasmussen Daily Tracking, CNN/ORC International, Newsweek/Daily Beast, Democracy Corps, Fox News Channel and NBC News/Wall Street Journal.
The latest Gallup poll asked approximately 3,050 registered voters: "Suppose the presidential election were held today. If Barack Obama were the Democratic Party's candidate and Mitt Romney were the Republican Party's candidate, who would you vote for Barack Obama, the Democrat or Mitt Romney, the Republican?"
Those who said they were undecided were further asked if they leaned more toward Obama or Romney, Gallup said.
Their leanings were incorporated into the results.
"Romney has not been ahead of Obama by more than 2 points since mid-May, and Romney's largest lead of 5 points occurred in April," Gallup Editor in Chief Frank Newport said in a statement.
Despite leading Romney in the Gallup poll, Obama's approval rating remains below 50 percent.