Jobs, a survivor of pancreatic cancer, said chief operating officer Tom Cook would take responsibility for day-to-day operations but he would continue to be chief executive and be involved in major strategic decisions.
"At my request, the board of directors has granted me a medical leave of absence so I can focus on my health," Jobs, 55, wrote in an email to staff published on a regulatory newswire. "I love Apple so much and hope to be back as soon as I can."
Apple shares tumbled 8.3 per cent in Frankfurt this afternoon although the US markets were closed for a public holiday.
Apple has been closely identified with the charismatic Jobs, who revived the computer maker's fortunes in 1996 after a 12-year absence from the company he co-founded.
Analysts said the effect on Apple's operations should be limited in the short term, since its product line-up was strong, but his absence would be a worry if it became prolonged. Cook ran day-to-day operations during Jobs's last absence in 2009.
"It wasn't expected. This will come as a surprise to Apple investors and definitely take some shine off the Apple stock," said Alexander Peterc, equity analyst at Exane.
"But even if Steve Jobs never returns to Apple, I would not expect a visible, tangible impact on how Apple is executing over the next couple of years."