Brisbane-based JTA - which has private clients in the Pacific and Australia, including some in Papua New Guinea's extractive industries - will now be called Abt JTA.
Abt said the subsidiary would continue to be led by JTA founder Dr Jane Thomson, as chief executive officer.
The acquisition is expected to provide opportunities for stronger joint bidding on contracts.
"Abt Associates and JTA International have complementary portfolios, values and mission-driven organisational cultures," Abt president and CEO Kathleen Flanagan said.
"The acquisition will provide new opportunities for growth and enable us to diversify our client base and compete on a broader range of projects."
Thomson said of the acquisition: "We are very excited to become a part of Abt Associates.
"There is a remarkable alignment of our values, our ethics, our subject matter expertise, and our commitment to top-quality work and client service."