Ballmer, 54, confirmed in a statement late Friday he is "selling some of his holdings of Microsoft stock to gain financial diversification and to assist in tax planning before the end of the calendar year."
He stressed that the move should not be seen as a lack of confidence in the Redmond, Washington-based software giant and he remained "fully committed" to the company.
Microsoft said Ballmer held approximately 408 million shares in Microsoft before the latest sale and plans to sell up to 75 million shares by the end of the year.
According to the SEC filing, Ballmer sold 21.87 million shares, 14.35 million shares and 13.12 million shares between Wednesday and Friday.
He currently holds 358.91 million shares in Microsoft worth some $9.64 billion, or 4.2 per cent of the company.
Ballmer is the second-largest Microsoft shareholder after founder Bill Gates, who sold three million shares of his own this week worth some $54.5 million.
"Even though this is a personal financial matter, I want to be clear about this to avoid any confusion," Ballmer said. "I am excited about our new products and the potential for our technology to change people's lives, and I remain fully committed to Microsoft and its success."
Microsoft shares closed on Wall Street at $26.85 on Friday.
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