The terms of the contract were not disclosed, but with each aircraft estimated at $250 million, analysts suggest the deal could top $1.5 billion.
The deal makes the U.A.E. the second nation in the Middle East, after Qatar, to buy the popular airlifter.
A joint statement issued by Boeing and the U.A.E. said the advanced C-17 planes would be delivered by 2012, with at least four provided a year in advance, in 2011. The deal was signed between Boeing and the U.A.E. air force.
Boeing, a global aerospace heavyweight contractor, is also expected to provide ongoing support for the planes, including material management and depot-management support.
The massive multiservice C-17 Globemaster III can carry large combat equipment and troops. It can also serve as important airlift support for humanitarian aid missions bound across large distances to small "austere" airfields around the world, Boeing said in a statement.
An unnamed U.A.E. military representative quoted by The Wall Street Journal said the country will use the planes to "perform a variety of humanitarian and strategic lift operation around the world."
He did not elaborate.
The deal is reported to have marked an unexpected increase in the initial order that the U.A.E. had considered for addition and upgrading of its existing fleet.
In February 2009 the U.A.E. announced its designs to purchase four C-17s along with 12 smaller Lockheed Martin C-130J tactical transport planes. The combined investment was estimated to be worth $3 billion.
There has since then been no explanation for the change in the order.
"The C-17 will give the U.A.E. the ability to perform a variety of humanitarian and strategic lift operations around the world in support of both national and international missions," Mohamed Al Mazrouei, U.A.E. major general staff pilot, said in the statement.
"These missions require us to be ready for any contingency at any time and any place, and the C-17 meets our requirements," he said.
Qatar was the first nation in the Middle East to opt for the airlift plane, which presently numbers 212 in service worldwide. Of them, 193 are owned by the U.S. Air Force, Boeing said in its statement.
The C-17 Globemaster III currently services 19 national air forces.
"Boeing is pleased that the U.A.E. Air Force has selected the C-17 to meet its airlift requirements for the 21st century," said Jean Chamberlin, Boeing vice president of global mobility systems. "The C-17 consistently posts mission capability rates that are among the best in the world, earning it high marks for its industry-leading quality and reliability."