It will spend $40bn (£24.8bn) a month on buying mortgage-backed securities, with no end date.
Interest rates are likely to stay “exceptionally low” until at least mid-2015, the Fed added.
US economic growth cooled in the second quarter, coming in at a tepid 1.7 per cent a year.
US data out today showed that first-time claims for unemployment benefit jumped by 15,000 last week.
James Knightley, senior economist at ING, said: "This outcome was broadly in line with market expectations given that the minutes to the August Federal Open Market Committee meeting stated that unless there was a 'substantial and sustainable' pick-up in activity 'many members' had felt further action would be 'warranted fairly soon'."