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.

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,

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