The 777-300ER (extended range) is a derivative of the 777-300 airplane. It is a C-market version and is equipped with raked wingtips for additional cruise altitude and range. The 777-300ER is powered by the GE90-115B turbofan, which is the world's most powerful high bypass ratio engine that develops higher thrusts than those used in the 777-200/-200ER/-300 airplanes, with a maximum thrust of 115,300 lbf (513 kN). The 777-300ER has an identical fuselage as the 777-300, but has a wider wingspan due to the raked wingtips. Other features of the 777-300ER include a new main landing gear, reinforced nose gear, and extra fuel tanks. It also has a strengthened fuselage, wings, empennage, and engine attachments. Its maximum range is 7,930 nautical miles (14,690 km), made possible by higher MTOW and increased fuel capacity. The 777-300ER can fly approximately 34 percent farther than the 777-300 with a full load of passengers and cargo. Following flight testing, aerodynamics refinements have reduced fuel burn by an additional 1.4 percent.

The first 777-300ER was delivered to Air France on April 29, 2004. The -300ER is the best-selling 777 variant, having surpassed the -200ER in orders in 2010 and deliveries in 2013.