Croatia has ordered another eight Sikorsky UH-60M Black Hawks, adding to four examples already in service.
The contract came into force on 13 March following the signature of a letter of acceptance by defence minister Ivan Anusic.
Deliveries will run until 2028, the Croatian defence ministry says. The US government is providing 51% of the funding for the helicopters, it adds.
Approving the acquisition in January, the US Defense Security Cooperation Agency valued the Foreign Military Sales purchase, including related equipment and services, at $500 million.
Croatia will use the Black Hawks for transport, medical evacuation, search and rescue, and special operations support missions.