The deal includes two aircraft for which the African carrier disclosed a commitment at the Paris air show in June.
At today's order announcement at the Dubai air show, Ethiopian's chief executive Tewolde Gebremariam confirmed that the aircraft were for additional capacity rather than replacement.
They will be delivered in 2018-19.
Flight Fleets Analyzer shows that Ethiopian has six 777Fs in service.
Asked about a potential 777X order by the Star Alliance carrier, Gebremariam replied: "We are still discussing it."
(Stephen Trimble - FlightGlobal News)