Arlington-based CACI International said Wednesday it has agreed to buy Six3 Systems, an intelligence contractor, for $820 million, marking the defense company’s largest acquisition.

The deal — CACI’s 57th acquisition — will move the firm further into the intelligence, cybersecurity and reconnaissance fields, areas considered likely to grow even as defense spending shrinks. McLean-based Six3, which has about 1,600 employees, is expected to report about $470 million in revenue this year.

The majority of Six3’s work is with intelligence agencies, and nearly all of the company’s employees have the highest security clearance, CACI executives said in a conference call with analysts Wednesday.

“It’s a strong price [but] it’s not as high a price as some of the other comparable companies sold [for] a few years ago,” said Bob Kipps, managing director of the McLean-based investment firm Kipps DeSanto. “It’s clearly a high-water mark in the industry this year, and it’s certainly a sign that there’s long-term confidence in the marketplace.”

CACI paid a significant premium for Six3, but the deal opens up new markets, said Michael S. Lewis, managing director of the Silverline Group, a consulting firm.

“It’s expensive when you compare them to a plain vanilla IT company . . . but Six3 wasn’t a plain vanilla IT company,” Lewis said.

As government spending declines, many defense contractors, including CACI, have made an aggressive effort to jump into more lucrative and growing markets.

CACI, which reported a 9.4 percent drop in its latest annual profit, abruptly replaced its chief executive this year, appointing Ken Asbury to the position. It also revamped its business-development office, hiring several new executives, in the hopes of becoming more competitive.

“We’re taking the long-term view of where the market’s going to go,” Asbury said Wednesday.

Six3 will become a CACI business group and retain its name as well as its leadership team, including chief executive Robert Coleman, Asbury said.

Six3 Systems was formed in 2009 by private equity firm GTCR in a partnership with Coleman, the former president and chief operating officer of Fairfax-based ManTech International.