The suit was filed in a Washington state court against Google and Kai-Fu Lee, who until Monday was the corporate vice president of Microsoft's Interactive Services Division.
"Today Microsoft filed a lawsuit against a former executive and Google regarding breach of Microsoft's employee confidentiality and non-compete agreement," Microsoft spokeswoman Stacy Drake wrote in an e-mail. "We are asking the court to require Dr. Lee and Google to honour the confidentiality and non-competition agreements he signed when he began working for Microsoft."
Google has emerged as a top rival for Microsoft, and several notable employees have left the software giant for Google in recent months. The company is seen as an aggressive rival to Microsoft in areas such as desktop search and e-mail. In addition, its services work well with any operating system.
Earlier on Tuesday, Google announced Lee's hiring and its plans to open a China research and development centre this quarter.
"Under the leadership of Dr. Lee, with his proven track record of innovation and his passion for technology and research, the Google China R&D centre will enable us to develop more innovative products and technologies for millions of users in China and around the world," Alan Eustace, Google engineering vice president, said in a statement.
Lee, an expert in speech recognition technology, founded Microsoft's China research lab in the late 1990s and worked at Silicon Graphics and Apple Computer before joining Microsoft.
A Google representative was not immediately available to comment on Microsoft's legal actions.
Google's public touting that it had hired Lee is in and of itself unique. The company rarely announces new hires, with CEO Eric Schmidt's hiring being a notable exception.
Though workers leave tech companies for rivals all the time, it's not uncommon for a dispute to end up in court, particularly when an executive has a contract with a noncompete clause. Microsoft has turned to legal channels before to pursue former employees who it felt were unfairly competing against the company.
Notably, the company sued when former executive Tod Nielsen and a number of ex-Microsoft employees went to work for Crossgain, a start-up that was focused on allowing business applications to run over the Web.
Crossgain eventually disassociated itself from a number of Microsoft workers that were still bound to noncompete agreements. Among the other ex-Microsofties who were forced to step down, at least temporarily, were Nielsen and Adam Bosworth, a founder of Crossgain.
Crossgain was swallowed up by BEA Systems in 2001, with both Nielsen and Bosworth joining the software maker. The two left BEA last year, and Bosworth joined Google.
A Microsoft lawyer said in an interview that Lee's move to join Google was a "particularly egregious" violation of the noncompete agreement that he had signed when he joined Microsoft.
"He has access to sensitive information, to trade secrets about our search technology and business plans and our China business strategies," said Deputy General Counsel Tom Burt. "He has accepted a position in direct competition with Microsoft in those areas."
Lee had been working most recently at Microsoft's Redmond, Washington, headquarters, focusing on new search technologies. He also served as a member of a company advisory board that concentrated on China-related strategies.
Burt said that Lee notified Microsoft of his plans on Monday and that Microsoft served him with legal papers later that day.
"There was no effort by Dr. Lee or Google to try and work out any kind of agreement," Burt said. "The combination of those factors meant that we really had no choice but to file this suit to protect our confidential information."
CNET News.com's Stefanie Olsen contributed to this report.
So other companies don't play fair either.
About time some-one else dealt out a bit of stick to Microsoft - after all it's what they appear to have been doing for some time - example is the EU case.
So what are they going to do about it - pick up their ball and go home?
I find this thing offensive - it's effectively a restraint of trade - isn't the US and Microsoft meant to champion free trade? Or is just lip service after all?