Microsoft published Ballmer's resignation letter on its website Tuesday along with a response from current CEO Satya Nadella thanking him and wishing him well.
Ballmer says he plans to hold on to his Microsoft stock and will continue to offer feedback on products and strategy. With 333.3 million shares worth $15 billion, Ballmer's 4 percent stake in the company makes him the largest individual shareholder.
"I bleed Microsoft - have for 34 years and I always will," Ballmer wrote. "I will be proud, and I will benefit through my share ownership. I promise to support and encourage boldness by management in my role as a shareholder in any way I can."
Ballmer stepped down as chief executive in February.
Nadella thanked Ballmer for his support during the transition period and used the opportunity to reiterate the company's new focus on mobile devices and cloud computing.
"Under your leadership, we created an incredible foundation that we continue to build on - and Microsoft will thrive in the mobile-first, cloud-first world," Nadella said.