Cooley, the Silicon Valley-based law firm that is growing rapidly in the Washington region, has reached an agreement to acquire D.C.-based firm Dow Lohnes effective Jan. 1, the companies announced today.

Cooley, which has about 700 lawyers in the United States and China — including 150 in Washington and Reston — plans to absorb 54 Dow Lohnes lawyers in the District and become one of the 30 largest law firms in the city.

Dow Lohnes managing partner John Byrnes said his firm had been in merger talks with several law firms since the summer.

“We looked at our strategic position, the needs of our clients and the changes in the legal marketplace, and concluded we needed a platform that’s broader and deeper and has a better geographic footprint,” Byrnes said. “Cooley really solved all those things for us. Their expertise in technology clients is very complimentary to our core industries.”

The legal trade publication American Lawyer had reported that Dow Lohnes, which was once one of the nation’s 200 largest law firms, had lost nearly half of its lawyers during the past three years. Its merger with Cooley will not include the 12 lawyers in Dow Lohnes’s Atlanta office, which recently experienced a spate of partner departures. At its peak, the Atlanta office had about 30 lawyers. The firm’s employees in Atlanta will have to find other jobs before the end of the year.

Cooley, best known for its work with technology companies, opened its Reston office in 1999 and a D.C. office six years later.

Dow Lohnes’s roots in the Washington area run much deeper: Formed in 1918 as a firm specializing in communications law, Dow Lohnes made a name for itself representing companies regulated by the Federal Communications Commission, such as telecommunications, radio and television providers. The firm also represents for-profit and nonprofit education companies and major universities, as well as cable companies and other media properties.

“Their practice areas overlay perfectly on ours,” said Ryan Naftulin, head of Cooley’s D.C. office. “Higher ed, communications and media are all strongly steering into the regulatory environment via technology. . . . Their regulatory capabilities make sense for our client base.”

Joseph Conroy, Cooley’s chief executive, said picking up Dow Lohnes is part of his firm’s “15-year march to diversify itself from a principally West Coast-based technology company firm while still retaining our roots in the technology space.

“We need the same breadth and depth on the East Coast as we have on the West Coast,” he said. “Washington is one of the most critical markets. This gives us instant credibility because of the deep history this firm has in this town. . . . It makes us a Washington player in a way we have not been before.”