"I see a combination of the Clippers, civic contribution, teaching and study taking a lot of time," said Ballmer in a letter to Microsoft CEO Satya Nadella made public by Microsoft on Tuesday.
"The fall will be hectic between teaching a new class and the start of the NBA season so my departure from the board is effective immediately," he wrote.
Ballmer's exit from the board is not surprising after 12 months of change at the head of the 39-year old software giant.
A year ago this week Ballmer surprised investors with his plan to retire from Microsoft, amid discontent at his leadership fueled by activist shareholder ValueAct Capital.
That caused five months of uncertainty at the top of Microsoft until Feb. 4, when veteran insider Nadella was appointed CEO and co-founder Bill Gates stepped down as chairman.
As Gates sells shares to fund his philanthropy, Ballmer has become the biggest individual shareholder in Microsoft, owning a 4 percent stake worth about $15 billion.
"I hold more Microsoft shares than anyone other than index funds and love the mix of profits, investments and dividends returned in our stock," Ballmer said in his letter to Nadella. "I expect to continue holding that position for the foreseeable future."
Microsoft shares were up 25 cents at $45.08 on Nasdaq.
(Reporting by Bill Rigby; Editing by Diane Craft and Chizu Nomiyama)