The drop in Obama's weekly average was driven by particularly low ratings near the end of the week, with record-low three-day rolling averages of 42 per cent for August 12-14 and August 13-15 polling, it said.
"Prior to this weekend, Obama's three-day low had been 44 percent. Additionally, Obama's disapproval rating reached 50 per cent for the first time in the August 13-15 average," it said.
Obama visited Florida's Gulf Coast over the weekend as part of a vacation with his family, while his comments about the construction of a mosque near ground zero in New York City became a much-debated topic in the news media and on the Internet.
"As is usually the case, the impact of these or other news events on the President's approval ratings is difficult to disentangle," it said.
Gallup said Obama's ratings in the low 40 per cent range are not unusual in historical context. All presidents since Lyndon Johnson have suffered ratings below 40 per cent at some point in their presidencies, with several falling below 30 per cent.
Presidents Carter, Reagan and Clinton were -- like Obama -- in the low 40 per cent range during August of the second year of their presidencies. President George W Bush left office in January 2009 with a 34 per cent approval rating, Gallup said.
Obama in Office: Low on ratings, but impressive
Obama plays, eats with friends on 49th birthday
Barack Obama to honour Sir Paul McCartney
Majority of Islam rejects extremists, says Obama
'US shouldn't do nation-building in Afghanistan'