The policy, known as quantitative easing and often abbreviated as QE3, entails buying $40 billion in mortgage-backed securities each month. The end date remains up in the air, as the Fed will re-evaluate the strength of the economy in coming months.
The Fed is wasting no time. The purchases begin Friday and are expected to add up to only $23 billion for the remainder of September.
The bond-buying policy "should put downward pressure on longer-term interest rates, support mortgage markets, and help to make broader financial conditions more accommodative," the Fed's official statement said.
Meanwhile, the Fed will continue its existing policy known as Operation Twist. Together the two programs will add $85 billion in long-term bonds to the Fed's balance sheet each month.
In addition, the Fed also indicated that it plans to keep short-term interest rates at "exceptionally low levels" until mid-2015. Previously, the Fed had forecast rates would remain low until late 2014.
The central bank's main objective is to lower interest rates and mortgage rates in particular. By keeping rates low, the Fed hopes to fuel more spending and eventually, more hiring.
The Fed has been trying to stimulate the economy for over three years now, and has exhausted its usual tool by keeping interest rates near zero since late 2008. Quantitative easing is an unconventional way of trying to lower rates further.
But given that the unemployment rate has remained above 8%, the Fed is still not satisfied.
"The weak job market should concern every American. It imposes hardship on people and a waste of human skills and talents," Fed Chairman Ben Bernanke said in a press conference later Thursday.
Last week, the government's jobs report showed hiring slowed substantially in August and the labor force shrank.
In its statement, the Fed indicated it will not only continue QE3, but also "employ its other policy tools" if the "labor market does not improve substantially."
Bernanke also admitted that the Fed alone is not strong enough to fix the job market.
"I want to be clear -- While I think we can make a meaningful and significant contribution to reducing this problem, we can't solve it. We don't have tools that are strong enough to solve the unemployment problem," he said.
The Fed's accommodative policies have been contentious from the start. Republicans often warn that as the Federal Reserve has expanded the money supply, it has set the economy up for rapid inflation in the future.
Meanwhile, economists expect the benefits to be minor, and the risks are uncertain. The first two rounds of quantitative easing lowered interest rates and fueled stock market gains, but banks haven't been eager to lend out money readily.
Banks are sitting on $1.6 trillion in reserves and credit standards remain tight following the financial crisis. Households continue to pay down debt, and are in no hurry to ramp up their spending.
That said, it's possible the Fed's move could help the housing market slightly. New construction and home prices have already started picking up recently, and should mortgage rates fall further, that could fuel a quicker housing recovery.
The QE3 move comes after Bernanke has repeatedly urged Congress to do more to support the recovery in the short term, while still addressing the country's debt problem over the long term.
But Congress has done little to heed his advice, and given it's an election year, they're not expected to act anytime soon. Economists often cite the threat of fiscal cliff as one of the key reasons businesses remain reluctant to hire new workers.
"We're looking for policymakers in other areas to do their part," Bernanke said at the press conference. "We'll do our part and we'll try to make sure unemployment moves in the right direction but we can't solve this problem by ourselves."
The Fed may have acted Thursday, partly to offset the drag from fiscal policy.
In implementing QE3, the central bank does not use taxpayer money to buy bonds. Rather, it expands the U.S. money supply and electronically credits banks with more funds.
Of the Fed's 12 voting members, Richmond Fed President Jeffrey Lacker was the only one to oppose Thursday's decision. He objected against the 2015 forecast and QE3. He has dissented at every Fed meeting since January.