Flack, best known for appearing on the X Factor spin-off show, is the 12th star confirmed for the BBC ballroom show.
She said: "Ever since I was a presenter on CBBC and used to see the Strictly judges walking around Television Centre, I have wanted to be on the show. I am so excited to get started and I just want to get through the first dance with no mistakes... plus I really want to master my 'dance face'!"
She joins other famous faces including Frankie Bridge, Judy Murray, Mark Wright and Pixie Lott in this year's show.
The final three celebrity contestants will be confirmed live on The One Show tomorrow.
The new series will be the first one without veteran host Sir Bruce Forsyth, with Tess Daly and Claudia Winkleman sharing the presenting job.
Flack tweeted: "Gahhhhhhh! I'm taking part in this years Strictly ...I haven't been this nervous since I entered the Norfolk Disco Dance championship 1992".