US Defense Secretary Leon Panetta has signed an order to send two Patriot missile batteries to Turkey over potential threats from Syria.
Up to 400 American personnel are being sent to operate them.
A spokesman for the defence secretary George Little said: "The purpose of this deployment is to signal very strongly that the United States, working closely with our NATO allies, is going to support the defense of Turkey, especially with potential threats emanating from Syria."
NATO-member Turkey has repeatedly scrambled jets along the countries' joint frontier.
It responded in kind when shells from the Syrian conflict came down inside its borders, fanning fears that the civil war could spread to de-stabilise the region.
The widely expected US move follows similar steps by Germany and the Netherlands.
Germany and the Netherlands also said they will send two Patriot batteries.
The three countries are the only NATO nations with the most modern type of Patriots.
Mr Little declined to say where the US batteries would be located and said the systems would be deployed to Turkey for an unspecified amount of time.
NATO approved Turkey's request for air defence batteries on 4 December in a move meant to calm its fears of coming under missile attack, possibly with chemical weapons, from Syria.
The Patriot system is designed to intercept aircraft or missiles. NATO says the measure is purely defensive, but Russia, Syria and Iran have criticised the decision, saying it increases regional instability.
Russia's Foreign Ministry spokesman Alexander Lukashevich has said that Moscow has not changed policy on Syria and will not do so.
The comments came after a Russian diplomat remarked that President Bashar al-Assad's opponents might win.
Mr Lukashevich said: "We have never changed our position and we will not change it."
He also said Russia was not involved in any talks on President Assad's fate or on getting him out of Syria.