"That's clearly much higher than we were expecting. We, like most people, were thinking like 5,000 to 6,000," Kirk Materne, technology analyst at Evercore Partners, told CNBC right after the announcement.
As a result of the dismissals, Microsoft said, it will incur pretax charges of up to $1.6 billion for severance and related costs over the next four quarters.
"We are moving now to start reducing the first 13,000 positions, and the vast majority of employees whose jobs will be eliminated will be notified over the next six months," Nadella wrote. "It's important to note that while we are eliminating roles in some areas, we are adding roles in certain other strategic areas."
Nadella explained the cuts, saying they "simplify the way we work to drive greater accountability, become more agile and move faster." Microsoft will also have fewer layers of management, he said.
About 12,500 of the cuts will result from the company's "synergies and strategic alignment" with Nokia Devices and Services, Nadella said. Microsoft announced that it would acquired Nokia in September 2013.
"This is a significant cut that Satya Nadella is trying to put his own stamp on the company. It also tells you that he's not going to let Nokia become a quagmire," Evercore's Materne said on CNBC's "Squawk Box."
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—By CNBC. Reuters contributed to this report.