Champion will anchor the network's new flagship morning show, which is set to debut in early 2014, The Weather Channel president David Clark said.
Based at The Weather Channel headquarters in Cobb County, Champion will also assume managing editor responsibilities at the network.
"With his commitment to understanding, predicting and explaining the power of weather in our lives, Sam perfectly embodies The Weather Channel brand's philosophy, 'It's Amazing Out There,'" Clark said.
Champion called his new post "a once-in-a-lifetime opportunity for me to do what I love most at a network that lives and breathes the weather."
He had been on that program since 2006, and joined ABC-owned local New York station WABC in 1988.
"For 25 years, Sam has been a vital member of the ABC News family," said ABC News president Ben Sherwood in announcing Champion's departure.
He said Champion's fellow meteorologist Ginger Zee will take over his weather responsibilities at "GMA" and across the news division. Zee became weather anchor for the weekend edition of "GMA" two years ago. Champion has been a member of the "GMA" anchor team as it seized the ratings crown from NBC's perennially front-running "Today" show.