By Ritsuko Ando

NEW YORK (Reuters) - U.S. communications equipment maker Juniper Networks Inc <JNPR.O> named senior Microsoft veteran Kevin Johnson as its new chief executive, saying he will start work in September.

Johnson, 47, replaces Scott Kriens, who had led the company for 12 years and will continue as chairman of Juniper's board, the company said.

Known as "KJ," Johnson was president of Microsoft's Platforms and Services Division, its largest business unit, and had spearheaded the software maker's failed pursuit of Yahoo Inc <YHOO.O>.

Microsoft announced Johnson's departure on Wednesday, alongside a reorganization that split the division into two: one focused on the dominant Windows operating system and the other focused on its lagging online business.

Juniper said Johnson was chosen while the company was searching for a chief operating officer, after Stephen Elop left in January to head Microsoft's business division. The decision by Juniper's board, including Kriens, to appoint Johnson as CEO was unanimous, said spokeswoman Sarah Sorensen.

She also said it would be up to Johnson to determine the still-vacant COO position.

Juniper makes most of its revenue from selling high-performance routers to communications service providers. Earlier this year, it expanded into the market for ethernet switches, which direct traffic on corporate data networks, moving into more direct competition with bigger rival Cisco Systems Inc <CSCO.O>.

Sorensen said Kriens, 50, would remain active in Juniper and oversee leadership development programs.

"Having been CEO for 12 years, he's at a point where he'd like to find a balance in his personal life," she said.

The reshuffle surprised some analysts, who were focused on the succession plan for Cisco, where CEO John Chambers eight years older than Kriens.

Johnson joined Microsoft in 1992 and worked his way up to being the head of worldwide sales. He was put in charge of Windows and charged with incorporating Microsoft's online services business with its traditional software groups.

He was in the inner circle of Microsoft executives who advocated a takeover of Yahoo. His departure is seen as a setback to the software maker's online business, which trails that of Google Inc <GOOG.O>.

Shares of Juniper fell 5.3 percent to $21.68. Microsoft shares fell 2.8 percent to $25.69.

Juniper is due to report quarterly results later on Thursday.

(Additional reporting by Tiffany Wu in New York and Daisuke Wakabayashi in Seattle, editing by Dave Zimmerman and Derek Caney)