The TV presenter posted a message on Twitter to her 200,000 followers explaining why she had chosen to make the change.
"As a news journalist & presenter I couldn't turn down this opportunity and a new challenge," she said.
"Thank you for your messages. I'm very excited to be joining Good Morning Britain. I have loved being at BBC Breakfast & will miss the team."
Susanna, 43, was officially unveiled as one of the stars of the revamped Good Morning Breakfast on Monday. When the programme replaces Daybreak in April, she will be joined by Charlotte Hawkins, who currently appears as Eamonn Holmes' co-host on Sky Sunrise.
ITV presenter Ben Shepherd and Sky Sports broadcaster Sean Fletcher complete the line-up.
Susanna joined BBC Breakfast in 2003 and became one of its main presenters when the show moved to Salford in 2012.
The confirmation that she had left the cooperation comes days after it was revealed that her 15-year relationship with the father of her three sons has come to an end.
Dominic Cotton is said to have become unhappy with her long absences from home while she juggled her job with her commitments to Strictly Come Dancing.