Jerry Yang, who co-founded Yahoo! nearly 17 years ago and had an ill-fated stint as chief executive, abruptly resigned on Tuesday from all of his positions with the struggling Internet company.

Yang, 43, a Web pioneer and one of the original dotcom billionaires, stepped down, effective immediately, from the board of directors of Yahoo! and from the boards of Yahoo! Japan and Alibaba Group Holding Ltd.

Yahoo! shares rose more than 3.5 percent to $15.99 in after-hours trading following the announcement that Yang was leaving the Sunnyvale, California, company.

Yahoo! did not name a replacement for Yang on the board and technology blog All Things Digital, citing sources close to the situation, said four other board members may also step down soon, including chairman Roy Bostock.

In a letter to Bostock, Yang, who held the title of "Chief Yahoo!," said "my time at Yahoo!, from its founding to the present, has encompassed some of the most exciting and rewarding experiences of my life. However, the time has come for me to pursue other interests outside of Yahoo!

"I am enthusiastic about the appointment of Scott Thompson as Chief Executive Officer and his ability, along with the entire Yahoo! leadership team, to guide Yahoo! into an exciting and successful future," Yang said.

Bostock, in a statement, described Yang as a "visionary and a pioneer, who has contributed enormously to Yahoo! during his many years of service.

"He has always remained focused on the best interests of Yahoo!'s stakeholders, including shareholders, employees and more than 700 million users," he said.

"And while I and the entire board respect his decision, we will miss his remarkable perspective, vision and wise counsel," Bostock said.

Yang co-founded Yahoo! in 1995 with David Filo and the company went public the following year.

Yang served as chief executive of Yahoo! from June 2007 to January 2009, during which time he notably turned down a $47 billion takeover bid from Microsoft, earning the ire of many shareholders.

Thompson, who was named CEO just two weeks ago, replacing Carol Bartz, who was fired in September, said Yang "leaves behind a legacy of innovation and customer focus for this iconic brand.

"Jerry has great confidence in the future of Yahoo!, and I share his confidence in the enormous potential of Yahoo! in the days ahead," Thompson said.

Since Bartz's departure in September, Yahoo!'s board has reportedly been looking at selling all or part of the company and Yang was seen as a fierce opponent of a breakup by some shareholders.

Yang owns 3.6 percent of Yahoo!'s shares and has a net worth of $1.1 billion, according to Forbes.

Analyst Jon Ogg of 247wallst.com said Yang's departure was "classified as a resignation" but "in reality this is a force-out."

"Needless to say, many investors are going to be glad to see Jerry Yang out of the way," Ogg said.

"Yahoo! is barely half of its market value today compared to when Microsoft Corporation made its buyout offer," he said.

Global Equities Research analyst Trip Chowdhry said Yang's departure was a "good move, but not enough."

"The current board has to be dissolved too," Chowdhry said, adding that it was "also out of touch with the trends impacting the Internet Industry."

Microsoft has reportedly been collaborating with private investors to assemble another multi-billion-dollar offer for Yahoo!

At least nine private equity firms are also reported to be eyeing Yahoo! and its global audience of 700 million monthly visitors to the company's various websites, including Yahoo! News, Yahoo! Finance and Yahoo! Sports.

Online commerce titan Alibaba is 43 percent owned by Yahoo! and Alibaba Group chairman Jack Ma has a long-standing offer to buy all or part of Yahoo!

Yahoo! has strong content and popular websites, but has been losing online advertising business to search giant Google, social networking king Facebook and specialized websites.