Couric, who has spent 15 years as part of the NBC morning news show, will become the first woman to solo anchor a U.S. flagship newscast.
"After listening to my heart and my gut -- which have served me very well in the past few years --I've decided that I will be leaving 'Today' at the end of May," Couric said.
In addition to her anchor job at CBS, she will also become managing editor of "The CBS Evening News With Katie Couric," contribute to "60 Minutes" and will anchor CBS News primetime specials, CBS announced on its Web site.
During her emotional farewell to her "Today" cohorts, Couric noted "once in a while, we get on each other's nerves."
But she turned to co-host Matt Lauer and said, "As Dorothy said to the Scarecrow, I think I'm going to miss him most of all."
Couric, 49, was near the end of her 4-year, $65-million contract with NBC. Her CBS deal is reported to be worth $13 million a year.
She will take over the "Evening News" from interim anchor Bob Schieffer in September.