Best known for playing Jane Beale in the Albert Square soap, the actress will start filming when the school drama relocates to Scotland next month.
She will play Scotland-born Christine Mulgrew, an English teacher appointed by headmaster Michael Byrne (Alec Newman) when he sets up the new school in the eighth series.
"I am thrilled to be joining Waterloo Road," Brett said in a statement. "I have watched it on and off from the start as my good friend Jill Halfpenny was in the first series.
"The character I will be playing is such a complex woman, with many issues, and it will be a real departure from what people are used to seeing me do.
"Not least because for the first time in my career I will be working in my native accent which I am really excited about."
The 42-year-old, who made her last EastEnders appearance in January, is competing in this year's Let's Dance for Sport Relief final later this month.
Waterloo Road's seventh run is currently airing every Wednesday on BBC One at 8pm.
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