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 the company’s CEO Wednesday, saying he can no longer handle the job.

The move appears to be the result of an unspecified medical condition for which he took an indefinite leave from his post in January. Apple’s chief operating officer, Tim Cook, has been named CEO.

In a letter addressed to Apple’s board and the “Apple community,” Mr. 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.”

Apple said Mr. Jobs was elected board chairman and Mr. Cook is becoming a member of its board.

The company said Mr. Jobs gave the board his resignation Wednesday and suggested Mr. Cook be named the company’s new leader.

The January leave was Mr. Jobs’ third medical leave over several years. He had previously survived pancreatic cancer and received a liver transplant.