Turkish Airlines has finalised an order for 15 Boeing 777-300ER aircraft worth $4.7 billion at list price.
The agreement, first announced as a commitment in October, also includes options for an additional five aircraft of the same type.
This order will add to Turkish's existing fleet of 12 777-300ERs and be used on its long-haul operations, says Boeing.
The Turkish flag carrier had previously said that three of the aircraft will be delivered in 2014, followed by seven in 2015 and five in 2016.
In early October, the carrier also signed an agreement with Airbus for 15 A330-300s and would have 27 of the type when the deliveries are completed in 2016.
Flightglobal's Ascend Online database shows that Turkish Airlines operates a fleet of 174 aircraft, comprising 12 777s, 21 A330s, seven A340s, 75 A320-family jets and 59 737s.