Nokia announced late Thursday that Stephen Elop, who up until now was president of the Microsoft Business Division, will become president and chief executive of the mobile-phone giant.

Elop will take the reins of the Finnish company after its current CEO, Olli-Pekka Kallasvuo, steps down Sept. 20 and remain on the board of directors. Kallasvuo has been with Nokia for 30 years, the company said.

In a prepared statement, Elop said he was “extremely pleased” to join Nokia, which boasts a dominant standing in the worldwide mobile market.

“It is the case in the technology world that there are critical moments where fundamental change takes place,” Elop said during a news conference Friday. “Today, right now today, we are going through a similar moment of fundamental change.

“If you think about the explosion in the use of all forms of mobile devices, including smart forms and tablets and everything else; if you think about the advent of the cloud-based computing strategies and the delivery of services from the Internet; or if you consider how all of us and our children are now communicating using all forms of social media. If you put all of those pieces together, you quickly recognize that these elements are conspiring to create a moment of fundamental disruption, and we are in that moment of disruption today in our industry, throughout the computing industry.”

“For Nokia, this moment of change represents a tremendous opportunity,” he added. “My role, as the leader of Nokia, is to lead this team through this period of change.”

Elop joined Microsoft in 2008 to run one of the Redmond-based company’s two biggest businesses; the Microsoft Business Division is responsible for Office. He previously was chief operating officer at Juniper Networks and held senior positions at Adobe after it acquired Macromedia, where he rose through the executive ranks.

Microsoft CEO Steve Ballmer sent the following e-mail to all employees:

I am writing to let you know that Stephen Elop has been offered and has accepted the job as CEO of Nokia and will be leaving Microsoft, effective immediately. Stephen leaves in place a strong business and technical leadership team, including Chris Capossela, Kurt DelBene, Amy Hood and Kirill Tatarinov, all of whom will report to me for the interim. The MBD business continues to grow and thrive, with 15 percent growth in the last quarter. It has been good to see the great response to Office 2010 and SharePoint 2010, the growth of our Dynamics business and the way we have been successful in extending all our MBD products and services to the cloud. I appreciate the way that Stephen has been a good steward of the brand and business in his time here, and look forward to continuing to work with him in his new role at Nokia. Please join me in wishing Stephen well. Steve

Microsoft did not say whether it has a replacement lined up.

Elop is the second division head to leave Microsoft in four months. Robbie Bach, president of the Entertainment and Devices Division, announced plans to leave in May — along with Xbox visionary J Allard. Chris Liddell left his position as CFO in November 2009, turning up the next month as CFO of General Motors.

Here’s a statement from Nokia Chairman Jorma Ollila:

The time is right to accelerate the company’s renewal; to bring in new executive leadership with different skills and strengths in order to drive company success. The Nokia Board believes that Stephen has the right industry experience and leadership skills to realize the full potential of Nokia. His strong software background and proven record in change management will be valuable assets as we press harder to complete the transformation of the company. We believe that Stephen will be able to drive both innovation and efficient execution of the company strategy in order to deliver increased value to our shareholders.

Elop said his first job, when he takes over on Sept. 21, will be to listen.

“In the weeks and months ahead, I have a great deal to learn about Nokia, but I also have a great deal to learn about Finland,” said Elop, who plans to move near the Finnish capital of Helsinki. “And that has already begun, as I’m very proud to say.”