San Francisco ? Steve Jobs, the mind behind the iPhone, iPad and other devices that turned Apple Inc. into one of the world’s most powerful companies, resigned as CEO on Wednesday, saying he can no longer handle the job but will continue to play a leadership role.

The move appears to be the result of an unspecified medical condition for which Jobs took a leave from his post in January. Apple’s chief operating officer, Tim Cook, was quickly named CEO of the company Jobs co-founded 35 years ago in his garage.

In a letter addressed to Apple’s board and the “Apple community,” Jobs said he “always said if there ever came a day when I could no longer meet my duties and expectations as Apple’s CEO, I would be the first to let you know. Unfortunately, that day has come.”

The company said Jobs gave the board his resignation Wednesday and suggested Cook be named the company’s new leader. Apple said Jobs was elected board chairman and Cook is becoming a member of its board.

Genentech Inc. Chairman Art Levinson, in a statement issued on behalf of Apple’s board, said Jobs’ “extraordinary vision and leadership saved Apple and guided it to its position as the world’s most innovative and valuable technology company.”

He said that Jobs will continue to provide “his unique insights, creativity and inspiration,” and that the board has “complete confidence” that Cook is the right person to replace him.

“Tim’s 13 years of service to Apple have been marked by outstanding performance, and he has demonstrated remarkable talent and sound judgment in everything he does,” Levinson said.

Jobs’ health has long been a concern for Apple investors who see him as an industry oracle who seems to know what consumers want long before they do. After his announcement, Apple stock quickly fell 5.4 percent in after-hours trading.

Jeff Gamet, managing editor of The Mac Observer online news site focused on Apple, said Jobs’ departure has more sentimental than practical significance.

“All Apple really has done is made official what they’ve been doing administratively for a while now, which is Tim runs the show and Steve gets to do his part to make sure the products come out to meet the Apple standard,” he said.