Semel's departure comes just two days after Yahoo revealed plans to lay off 1,000 employees as part of an effort to revitalize a company that analysts say strayed from its profitable strengths while Semel was at the helm.
Yahoo co-founder Jerry Yang replaced Semel as chief executive in June to shore up the California firm's profits and stock price.
Semel was relegated to a post as non-executive chairman of the Yahoo board.
Yahoo announced that Thursday would be Semel's last day on the board of directors.
Semel told Yahoo executives several months ago that he wanted to leave the board and worked with Yang and others to find a replacement.
Board member Roy Bostock is replacing Semel as non-executive chairman.
"It has been an honor for me to work with Yahoo's incredibly talented people over the past seven years, and I'm proud of all that we've accomplished during that time," Semel said in a written release.
"With the company moving forward under new leadership, I believe