Billionaire Carl Icahn, who joined Yahoo! Inc.’s board last year after threatening a proxy fight, stepped down from the position, saying the company no longer needed an activist director.

Icahn said his attention is now focused elsewhere and that he doesn’t have enough time to devote to Yahoo!, according to a letter to the board that he released today. His resignation took effect immediately.

Icahn had sought to take control of the board and oust then-Chief Executive Officer Jerry Yang last year, after the company spurned a $47.5 billion acquisition offer by Microsoft Corp. To ward off a proxy fight, Yahoo! gave board seats to Icahn and two of his candidates in August 2008. Since Icahn became a director, Yahoo hired Carol Bartz as CEO and forged a search-engine partnership with Microsoft.