NEW YORK - ABC News' Martin Bashir is leaving his post at "Nightline" to be an anchor for MSNBC and a contributor to "Dateline NBC," the network announced Thursday.

Beginning in September, Bashir will anchor a daily afternoon program on MSNBC in addition to reporting for "Dateline NBC."

In an email to his "Nightline" colleagues, Bashir said he plans "to work in both daily and long-form journalism" at his new job.

MSNBC president Phil Griffin said Bashir "fits in with what MSNBC wants to be: He's smart, original and thoughtful."

Bashir joined ABC's newsmagazine "20/20" in 2004. Before that, he was a London-born journalist who won attention for documentaries including "Living With Michael Jackson," which aired on ABC in 2003, and an exclusive interview with Princess Diana in 1995.

Also on Thursday, ABC News announced that Bill Weir, who co-anchors the weekend edition of "Good Morning America," will join "Nightline" alongside current co-anchors Terry Moran and Cynthia McFadden. They, with Bashir, made up the original three-host team that replaced Ted Koppel in 2005.

In an email to staff Thursday, ABC News president David Westin said Weir "has done some of our most innovative reporting from every conceivable venue, both as co-anchor of weekend GMA and for all of our other programs and platforms."