Steven Wadsworth has unexpectedly ankled as president of Disney Interactive Media Group.

One of Hollywood’s respected executives in the online entertainment biz, Wadsworth had essentially overseen the Mouse House’s online and gaming efforts since 1999, when he became president of Disney’s interactive group.

Disney Interactive Media Group runs Disney.com, EPSN.com, ABC’s branded websites and the company’s gaming efforts.

Wadsworth’s replacement will be announced shortly, Iger said in a statement. One candidate being mentioned by many is John Pleasants, CEO of Playdom.

Wadsworth alerted his staff of the resignation Thursday night through an email. “I have been thinking about this for quite some time, and while it is difficult to leave a great company, an exciting business and a wonderful group of people, my desire and excitement to pursue other opportunities is too great to ignore,” he wrote.

In an official statement, Wadsworth relayed similar sentiments: “While there is never a good time to leave a great role at a great company, for many reasons now is the right time for me to move on,” he said. “I am extremely proud of the huge progress we’ve made at Disney Interactive Media Group, and am confident that the business is well-positioned for continued significant growth.”

While much attention has focused on the changes at Disney’s film division and more recently ABC, as well, the company’s interactive unit has also been going through a corporate overhaul lately.

After a series of mergers over the years and struggles with launching a profitable videogame division and replicating the success of ClubPenguin.com with online worlds based around its Disney Fairies franchise, starring Tinker Bell and her flying friends, as well as the “Pirates of the Caribbean” films, Disney chief Robert Iger is now having the group target the booming social gaming business.

In July, it picked up Playdom, a maker of social games for sites like Facebook, for $563 million and mobile gamemaker Tapulous before that.

“We’re all aware of the rapid growth of social networks and the huge popularity of games on them,” Iger told analysts while reporting the company’s third-quarter results, citing that the market for such games will grow more than 30% a year. “We feel it’s essential for us to have a presence. We now have a diversified games business to meet consumer interests.”

That business includes rolling out the high-profile console vidgame title “Epic Mickey” later this year, as well as August’s revving up of the virtual “World of Cars Online.”

While it will remain in the console business, creating mostly Disney-branded games for Nintendo’s Wii, Microsoft’s Xbox 360 and Sony’s PlayStation 3 devices, Disney will devote more resources to the online gaming biz.

Yet while ClubPenguin.com has proved a hit with kids, Disney hasn’t been able to replicate that success with separate online worlds for franchises like its Disney Fairies, featuring Tinker Bell and friends, and “Pirates of the Caribbean.”

Still, revenue from interactive media rose a whopping 74% to $197 million during the third quarter. Losses in the division were cut by $10 million to $65 million vs. the same quarter a year ago.

“During his long tenure, Steve has transformed our Disney digital business in a rapidly changing landscape,” Iger said in a statement. “He leaves with only our best wishes for continued success in whatever challenge he takes on next.”

Wadsworth joined Disney Online as senior VP and chief financial officer in 1995, and was director of business planning at Disney Consumer Products before that. Wadsworth had worked at Disney since 1993.