The new Net venture will air three times a week on ABCNews.com.
Donaldson will continue to file reports for 20/20 and other ABC News broadcasts and will serve as a substitute anchor for the newsmagazine as needed. He'll also continue as cohost of This Week With Sam Donaldson and Cokie Roberts.
"I am very excited about the opportunity to try some new things, and it is really only possible by stepping away from my regular duties at the White House and the weekly assignment of anchoring 20/20," Donaldson said.
Charles Gibson will replace Donaldson as coanchor of the Wednesday edition of 20/20, ABC said.
In addition, ABC confirmed yesterday that Barbara Walters will anchor the Friday edition of 20/20 solo once Hugh Downs steps down later this month. Connie Chung and Jack Ford will anchor the Monday installments of the program.
Rotating anchors and correspondents will be used for the new Thursday edition of the newsmagazine. Branded 20/20 Downtown, it will broadcast from remote locations.