Aeromexico first announced its plan to order 787-9s in July when the carrier said it would order up to 90 737 MAX jets as well.
The Dreamliner order is valued at $1.46 billion at list prices and includes rights to four additional 787s.
With this 787 order, Aeromexico has a total of 19 Dreamliner aircraft on order with Boeing. The carrier expects delivery of its first 787 next summer.
"These additional 787s will help Mexico's largest airline expand its international presence with a stellar product for its valued customers," Van Rex Gallard, a sales vice president for Boeing Commercial Airplanes, said in a statement.
Fifty-eight customers have placed orders for more than 800 of Boeing's mostly composite, fuel-efficient 787 aircraft.
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