The order could breathe new life into an aircraft Boeing has said it may cancel.
The firm warned in June that its iconic 'Jumbo Jet" plane faced falling orders and pricing pressures.
The purchase significantly adds to Boeing's order book when it and rival Airbus have struggled to sell wide-body aircraft.
The UPS order, which includes an option for 14 more 747s, reflects growing demand for air freight, chief executive David Abney said.
"We have the demand and these aircraft will allow us to handle the demand on a very scheduled basis," he said.
Slow sales of the 747-8, the latest version of the four-engine plane that first flew in 1969, prompted Boeing to recently cut production to six a year and to warn in July it might stop making the plane.
Similar-sized two-engine planes that are more fuel-efficient have overtaken the 747 as passenger planes.
Sales have also slowed for Airbus's large, four-engine A380 passenger jet.
But the 747-8 freighter's hinged nose gives it an ability to carry large cargo.
UPS plans to use its 747-8s on important "trunk routes... connecting Europe to Asia, Asia to the US", Mr Abney said.
It will use older 747s and other planes to replace smaller planes, adding capacity.
"It's really a cascading effect that is more than the sum of its parts," Mr Abney said. "We may be adding only 14 at this time but... there's going to be multiples of larger aircraft with more capacity and much more locations."
"What is important is this is net add," he added. "In some places you're buying just to replace aircraft. This is not. This is a growth strategy. This is about opportunity.... We're going to put these planes to work as soon as we get them."
Courier firm United Parcel Service has ordered 14 Boeing 747s in a deal worth $5.3bn (Â£4.4bn) at list prices.