Apple CEO Tim Cook speaks on stage during an announcement of new products at Apple Park in Cupertino, Calif., on Sept. 9. Godofredo A. Vásquez/AP hide caption

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Godofredo A. Vásquez/AP

Tim Cook is stepping down from his role as CEO of Apple.

In a letter posted publicly on Monday, Cook said Apple will have a new leader starting in September.

"I will be transitioning into a new role, leaving the CEO job behind in September and becoming Apple's executive chairman," Cook, 65, said.

Cook added the company's new CEO will be John Ternus, an engineer who has been with Apple for 25 years. According to Apple's web site, Ternus has overseen hardware engineering on several of its products, including iPads and AirPods, Macs, the Apple Watch, and the iPhone.

"I think this is definitely a signal that we're doubling down on hardware. And frankly, the hardware unit has been incredibly successful," said Jo-Ellen Pozner, an associate professor at Santa Clara University's business school. 

Cook inherited the top job from Apple's visionary cofounder Steve Jobs. In many ways Cook was the opposite of Jobs' charismatic leadership style, carving instead a reputation for himself as a calm and mild-mannered leader.

Cook leaned into his operations experience, expanding and transforming supply chains and launching a line of Apple-designed chips for the company's devices.

Cook also moved Apple beyond hardware into services, introducing a slate of subscription products including Apple News, Apple TV and Apple Pay. Today, those represent sizable revenue streams for the company.

During his term as CEO, Apple dominated the global smartphone market with its iPhone and the company's value skyrocketed to around $4 trillion today.

Cook was also the first CEO of a Fortune 500 company who publicly came out as gay in a personal essay in 2014. It was widely hailed as a welcome moment for such a prominent CEO to lend his voice and be a "role model" for LGBTQ youth.

In his letter to the Apple community on Monday, Cook wrote in personal terms about his time at Apple and how much he cherished starting every morning of his last 15 years by opening his email to read notes from Apple users about how the company's products had touched their lives — from the "perfect selfie you captured at the summit of a mountain that seemed impossible to climb" to the mom "saved by her Apple Watch."

He said he felt gratitude to be "the leader of a company that ignites imaginations and enriches lives in such profound ways it defies description."

Cook's transition comes a time of great upheaval for the tech industry caused by the artificial intelligence revolution. Apple hasn't been a leader in this area. Earlier this year, Apple turned to Google's AI prowess to power its products including iPhone's virtual assistant Siri.

Under Cook, Apple also became heavily dependent on China as a manufacturing base. It has pivoted and is attempting to diversify its supply chain, but experts say it is a vulnerability that will take years to mitigate.

"They have spent so long getting so focused on China that it is taking them a very long time — and will take them probably decades — to not be completely reliant on China," said Jason Snell, who writes about Apple at Sixcolors.com and MacWorld.

Apple is a financial supporter of NPR.