The new orders makes Etihad the world's largest customer of the fuel-efficient 787-9 with a total of 41 on order.
It also ordered two Boeing 777 Freighters, taking the number of 777s in its fleet to 21.
It was reported last month that Etihad had agreed the deal with Boeing, and the two parties steered clear of announcing the purchase at the Dubai Airshow, which was dominated by Etihad's Gulf-based rivals Emirates and Qatar Airways.
Etihad said on Monday that it also had options and purchase rights for 25 more 787s.
"Both the 787 Dreamliner and the 777 Freighter offer highly attractive operating economics and will facilitate our global expansion plans by allowing us to transport passengers and cargo into new markets from our hub in Abu Dhabi," Etihad Airways chief executive James Hogan said in the statement.
The 787-9 will be initially used on Etihad's routes to destinations including Dublin, Frankfurt, Kuala Lumpur, Beijing, Nagoya, Delhi and Istanbul, the carrier said.
Etihad had a muted presence at the airshow in Dubai last month, where more than USD$62 billion in civil aerospace and military orders were placed.
Emirates ordered 50 wide-body 777s at the show in a deal valued at USD$18 billion, which was the Boeing's largest-ever single-order deal.