(Reuters) - Barnes & Noble Inc said on Tuesday Chief Executive Ronald Boire would leave, after less than a year at the helm, as its board had decided he was not a "good fit" for the bookstore chain operator.

Executive Chairman Leonard Riggio would postpone his retirement and would assume Boire's duties, along with other executives, the company said.

Riggio, who is also the company's founder, was scheduled to retire in September following the annual shareholder meeting.

Barnes & Noble said its board had determined that "it was in the best interests of all parties for (Boire) to leave the company", but did not provide further details.

Barnes & Noble spokeswoman Mary Ellen Keating declined to comment beyond the company's statement.

Sales at Barnes & Noble, which operated 640 bookstores in the United States as of April 30, have been falling for the past two years due to rising competition from Amazon.com Inc.

Amazon's Kindle tablets and rich reading content have weighed on demand for Barnes & Noble's e-reader Nook.

Sales in the Nook business fell 20 percent in the fourth quarter ended April 30, as the company shut its Nook UK, app and video businesses.

The company said on Tuesday it would continue to work on its strategic initiatives, which includes winding down its Nook business and expanding its product catalog to include hot-selling items such as adult coloring books, art supplies, music vinyls and toys.

Barnes & Noble said it would immediately start the search for a new CEO to replace Boire, who joined from Sears Canada Inc in September last year.

(Reporting by Ankit Ajmera and Anya George Tharakan in Bengaluru; Editing by Maju Samuel and Sriraj Kalluvila)