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

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.

Apple’s chief operating officer plans to hand the role off this month and retire later this year after roughly a decade as the tech giant’s No. 2 under Chief Executive Tim Cook.