"David has been a tireless advocate for ABC News, effectively guiding the group through some of the most seismic industry, and divisional, changes imaginable," said Anne Sweeney, Disney-ABC Media Networks co-chairwoman, in a staff memo to ABC staff.
Westin, who just turned 58, also convinced Diane Sawyer and Charles Gibson to co-host "Good Morning America" a decade ago.
Westin announced his resignation to staff on Monday, saying he wants to pursue other opportunities. He has agreed to stay on through a transition to a successor.
"I've always admired those few who know when it's time to move on. This is the right time for me," Westin said in a staff memo.
Westin's successor will be appointed soon, Sweeney said.
Westin, a corporate lawyer with little background in the news business, was regarded with suspicion by many at ABC when he took over in March 1997 from television legend Roone Arledge.
Westin's most embarrassing moment came in 2000, when the network sent actor Leonardo DiCaprio, instead of a journalist, to interview President Bill Clinton about global warming. He was occasionally criticized for a deliberate decision-making style that left some of his broadcasts going through lengthy periods with substitute hosts.
He is a particular champion of Stephanopoulos, who has grown comfortable as a journalist when some questioned whether he could make the switch from politics. He also has promoted the investigative work of Brian Ross.
"While it will be sad to see David leave," Sweeney wrote, "his desire to pursue other professional endeavors is understandable, and commendable."
The Associated Press contributed to this story.
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