Yahoo! said co-founder Jerry Yang resigned from the board and all other positions, two weeks after the company announced new leadership under chief executive Scott Thompson.
The shares rose in extended trading.
Yang, who started Yahoo! in 1995 with David Filo, also left the boards of Yahoo Japan and Alibaba Group Holding, both of which Yahoo! has stakes in, effective today, Yahoo! said.
The company hired Thompson, former president of EBay's PayPal unit, on January 4, asking him to complete a strategic review of Yahoo!'s options that began in September.
"Jerry's thrown in the towel. He founded the company - this is his baby," said Colin Gillis, an analyst at BGC Partners LP in New York, who rates the stock a "hold" and doesn't own it. "The strategic position hasn't changed."