Mr. Williams was long considered a leading candidate to replace Tim Cook as the company’s next chief executive.

Sabih Khan, Apple's senior vice president of operations, will assume the COO role later this month as part of what Apple called a long-planned succession. Williams will continue reporting to CEO Tim Cook in the interim while overseeing the Apple Watch,

Apple Inc. Chief Operating Officer Jeff Williams is retiring as the company’s longtime No. 2, marking a major changing of the guard at an already tumultuous time for the iPhone maker.