The ex-politician is the first name to be officially revealed for the hit ballroom dancing show.
He told Chris Evans on his Radio 2 breakfast show that he was “scared to death”.
Mr Balls, 49, lost his Morley and Outwood seat in May last year in one of the greatest casualties of the 2015 election.
He said appearing on the BBC1 show was a “dream come true” but admitted he thought that life in politics was “much easier than this is going to be”.
Mr Balls said his wife, Yvette Cooper, the Labour MP for Pontefract and Castleford, supported his decision to join the show.
And he said he just hoped that he would get through the first two weeks.
He said: “Making a speech in Parliament seems a piece of cake compared to this, but hopefully I can stick around for a couple of weeks, and have a lot of fun in the process.
“Strictly is a firm favourite in our living room, and it feels like a dream to be going on myself, especially for Len’s (Goodman) last series. I’m just hoping it doesn’t turn into a nightmare too quickly!”
Mr Balls, who has competed in three marathons, said being on the dance show was part of his “post-politics bucket list” and that he could “do with losing half a stone”.
He joked that while he had been embarking on sensible endeavours such as learning the piano following the last general election, Strictly was outside his comfort zone.
“I’ve done sensible things. I learned the piano. I ran marathons. I always thought I wasn’t going to get a motorbike to go across the Sahara.
“I have a feeling that going on Strictly is a bit more like getting on a motorbike to go across the Sahara than practising the piano,” he said.
“I’ve taken the view, if you’re going to have a midlife crisis you should make sure you plan it well and enjoy it.”
The ex-cabinet minster admitted: “I have got to make sure I get fit in the next couple of weeks. From the beginning of September I’m definitely going to do 12 hours a week training.”
Mr Balls, who was famously spotted on the dancefloor at the Labour Party conference in 2014 doing a Gangnam Style dance, said joining the show was “quite an exposed thing to do” but that his family “have been massive fans for years”.
When he confided in his close friends and family about the show, he had expected them to say “Are you mad? What are you doing?” but their reaction was “What a great idea”.
He told Evans: “I spent 20 years in politics. I was a cabinet minister doing serious things. I’d like to do serious things again and get a full-time job again. In the meantime, I want to have some fun.”
He said his politician wife was a much better dancer than him.
“She kept saying ‘I’d like to do it ... you’ve got to do it. How can you say no?’ If she’d said ‘You’re mad’, I wouldn’t have done it.
He added: “My only hope is that it stays a dream and doesn’t become a nightmare.”
Other ex-politicians who have appeared in the ballroom show include Edwina Currie and Ann Widdecombe.
Former business secretary Vince Cable took part in the Christmas special in 2010, but Mr Balls will be the first male politician to appear on the main Strictly show.
Head judge Len Goodman has previously announced the next series of Strictly will be his last and ITV is launching a new big-budget rival to the BBC hit, set to launch in 2017.