According to the Gallup poll, conducted in August, 40% of Americans approve of Bush's job performance while 56%disapprove.
It is the worst result for the president since he took office in January 2001. Bush was re-elected in November.
The previous low score was last month, when 44% of Americans polled between 25-28 July said they approved of Bush's performance.
His previous worst disapproval figure was in a 24-26 June poll, when 53% had a negative opinion of the US leader.
The new poll "reflects further erosion in President George W. Bush's job approval rating, continuing the slow but steady decline evident throughout the year so far", according to Gallup.
Bush's three-poll average has fallen from 46% in May to 43% in August.
Compared to the average of other presidents during August of their second terms, Bush is only ahead of Richard Nixon, whose August average was 34%.
The survey was conducted by telephone among 1007 people between 22-25 August.
It has a three-percentage-point margin of error.