CACI International has acquired L-3 National Security Solutions (NSS) for around $550m.
NSS is a prime mission partner for the Department of Defense (DoD), intelligence agencies, and the US Federal Civilian Government.
The business is expected to help CACI expand its presence in enterprise IT and intelligence services, as well as other key market areas.
Based in Virginia, NSS offers strategic solutions, services and products to national security agencies and DoD critical missions in the US.
The company employs around 4,000 professionals worldwide, of which 90% possess security clearances.
According to CACI, the deal is expected to add around $1bn of annual revenue to the company.
CACI executive chairman and board chairman Dr Jack London said: "We welcome the highly skilled and experienced employees from National Security Solutions to CACI's team of talented and dedicated professionals.
CACI president and CEO Ken Asbury said: "CACI's acquisition of National Security Solutions is of great strategic value to our company.
"Our collective strengths and capabilities will enhance CACI's pursuit of higher value opportunities, provide a significant competitive advantage in pursuing and priming larger opportunities, and give us outstanding credentials to compete for and win next-generation IT programs across the federal marketplace."
CACI offers information solutions and services to the national security missions, as well as for intelligence, defence and federal civilian customers.