Lewis, who did not return phone calls, said in a statement that NPR's demands on his time were the reason for his decision.
Meanwhile, Edgar Woolard, an Apple board member, said that the qualities the company's directors are seeking in new board members will become apparent as they are named.
Lewis was one of six outside directors sitting on Apple's board. Apple said it has four open board seats, and will now seek a replacement for Lewis.
Lewis's departure from the board comes on the heels of the ouster of Apple's chief executive Gilbert Amelio earlier this month.
It also falls just as shareholder activist group CalPERS, the California Public Employees' Retirement System, is planning to reschedule a meeting with Apple to discuss its concerns about the troubled computer maker's lagging financial performance and stock price. The nation's largest public pension fund has expressed a lack of confidence in the computer maker's board members.
CalPERS has said that it thinks Apple's board members are stretched for time--with some sitting on numerous other boards--and has pointed out that the directors did not hold a sizable stake in the company.
At Apple's annual shareholders meeting earlier this year, CalPERS voted against reelecting the company's board of directors to another term.
"The idea of voting against the directors is more about sending a message," said Brad Pacheco, a CalPERS spokesman, in an earlier interview.
He said that CalPERS will vote against the board in the hope of eliciting change, rather than put its own slate of directors before investors at a company's annual shareholders' meeting.
CalPERS put Apple on its top ten list of underperforming companies in February.