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

Having worked at Apple for nearly three decades, Williams has played a significant role in introducing the iPod and iPhone programs, leading the development of the Apple Watch, and overseeing the design team,

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.