The company published a letter Tuesday from Ballmer to current CEO Satya Nadella.
In the letter Ballmer points to his new ownership of the LA Clippers among a growing list of other commitments that he says will take up most of his time going forward.
“Given my confidence and the multitude of new commitments I am taking on now, I think it would be impractical for me to continue to serve on the board, and it is best for me to move off,” wrote Ballmer. “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.”
The company also released Nadella’s response to Ballmer.
“While your insights and leadership will be greatly missed as part of the board, I understand and support your decision,” wrote Nadella.
Ballmer started with Microsoft 34 years ago and served as CEO for most of the last decade before stepping down 6 months ago.