"I see a combination of the Clippers, civic contribution, teaching and study taking a lot of time," Ballmer said a letter to current CEO Satya Nadella. "I have confidence in our approach of mobile-first, cloud-first, and in our primary innovation emphasis on platforms and productivity and the building of capability in devices and services as core business drivers."
He added that he intends to hold his large Microsoft stock position for the "foreseeable future."
Ballmer, who retired when Satya Nadella took over as CEO in February, recently bought the Los Angeles Clippers basketball team for $2 billion, and plans to devote much of his energy to that project.
The former CEO said that his "confidence" and "multitude of new commitments" would make it "impractical" for him to continue to serve on the board, especially as he will begin teaching a new class on top of the start of the NBA season.
Back in January, Re/code had reported that Ballmer would likely step down from the board once Nadella became CEO.
—By CNBC.com staff. Reuters contributed to this report.