The move puts troops on the ground in time for the increase in fighting that usually comes with warmer weather, and in time for national elections to be held in Afghanistan in August.
Meanwhile, the office of Hamid Karzai announced that Obama spoke to the Afghan president of Tuesday - the first time since the US presidential inauguration last month.
The two leaders spoke on the telephone about security issues and the Afghan presidential elections, Karzai's office said on Wednesday.
The troop increase is the first instalment of a larger influx of US forces that Obama was widely expected to announce after entering office in January.
Nato-led International Security Assistance Force (Isaf) troops in Afghanistan after deployment of 17,000 extra US soldiers:
US: 55,000 (south)
UK: 8,600 (Kabul, Kandahar, Helmand)
Germany: 3,500 (north)
Italy: 2,880 (west and capital Kabul)
Canada: 2,800 (south)
Netherlands: 1,650 (south)
France: 1,515 (Kabul)
Poland: 1,100 (mobile)
Australia: 1,070 (south)
Other significant deployments:
Denmark: 780 troops
Spain: 740 troops
Turkey: 675 troops
Romania: 535 troops
Hamish MacDonald, Al Jazeera's correspondent in Afghanistan, said that Obama's announcement on the troops' deployment had come later than expected.
"I think it is important to point out that there were expectations that this announcement would be made much earlier," he said.
"It has been widely reported that when Obama had his first meeting with the US senior military leadership, he was not satisfied with the strategy that they presented for the use of those additional troops.
"He sent them away, we are told, to look into a more convincing strategy and told them to come back with a request again. Only then would he announce the deployment."
The US administration is currently reviewing its policy in Afghanistan, the results of which are expected to be released in April, although Obama said the increase in troops would not "pre-determine" the outcome of the review.
Al Jazeera's Rob Reynolds says that for Obama to wait until April for a troop increase would be to risk a further increase in violence, which is why he acceded to his commanders' request.
However, he says the Pentagon may still be a little wary of too many soldiers in Afghanistan – many in the US are warning Obama not to go down the same route as Lyndon Johnson, the former US president, who found himself in a quagmire in Vietnam.
Analysts say US troop build-up in Afghanistan could reach up to 60,000 troops from current levels of 38,000 in the coming months.
Obama has previously criticised the Afghan government for seeming "detached" from the problems the country faces.
He told Canadian television on Tuesday that Afghanistan remained "winnable" despite a rise in deadly attacks by Taliban fighters, but said force alone would not guarantee victory.
"I'm absolutely convinced that you cannot solve the problem of Afghanistan, the Taliban, the spread of extremism in the region solely through military means," he told CBC.
"We're going to have to use diplomacy; we're going to have to use development."
Tensions have also increased after a number of US military operations led to civilian deaths.
A United Nations report says the number of civilians killed in the Afghanistan conflict last year jumped by 40 per cent to more than 2,100.
The US sent troops into Afghanistan in October 2001 following the September 11 attacks the same year, in a bid to destroy the al-Qaeda network and oust the Taliban from power.