Islamic state crisis: US troops sent into Iraq's Anbar
A Pentagon spokesman said the troops would examine site facilities and prepare support for Iraqi forces.
The deployment is the first time US troops have been sent to Anbar since US air strikes against IS began in August.
US forces left Iraq in 2011, but President Barack Obama said last week he was sending 1,500 as advisers.
Anbar, now largely under the control of Islamic State jihadists, was previously a base for al-Qaeda in Iraq and a hotbed of resistance to US forces during the years after 2003.
A spokesperson for the US military's Central Command told Reuters news agency the 50 troops would be stationed at the Ain al-Asad air base, the largest in Anbar.
Colonel Patrick Ryder added that though the troops were operating in an advisory capacity, they would defend themselves if attacked.
"A portion of the group consists of force protection personnel and any weapons US forces possess are for force protection requirements," he said.
'New phase'
On Tuesday, Iraqi forces successfully recaptured the centre of Baiji, home to Iraq's largest oil refinery, from IS militants, a senior army commander told state television.
General Abdul-Wahab al-Saadi said troops had retaken Baiji's local government headquarters and police station.
However a second military official told the Associated Press that intense fighting was continuing elsewhere in the town.
On Sunday, Mr Obama said the fight against IS was entering a "new phase" after four months of air strikes.
His decision to deploy 1,500 extra troops, at a request from the Iraqi government, will almost double the current US contingent in Iraq.
"Rather than just try to halt IS's momentum, we are now in a position to start going on some offence," the president said.
However Mr Obama has been adamant that US troops, which he withdrew from Iraq in 2011, will not resume combat, insisting that they are there to train and advise.
IS has taken over large parts of Anbar province as it expands its territory, currently about a third of both Iraq and Syria.
A US-led coalition has launched more than 400 air strikes on the group in Iraq since August, and more than 300 across the border in Syria.
The strikes have destroyed hundreds of the group's vehicles and several of its bases, but Islamic State has continued its campaign to establish a caliphate.
On 2 November, officials in Anbar said IS militants had killed at least 322 members of a Sunni tribe who had tried to resist the jihadists - but tribal leaders told the BBC the real toll was more than 630.
BBC
