She has agreed a deal with publisher Ebury and the book has a working title of Behind the Black Door. The book deal, believed to be worth six figures, will see Brown document her years spent living at Number 10.
The Browns left Downing Street last month, when Labour conceded power to the Conservative-Liberal Democrat coalition.
Brown confirmed she would be writing her memoirs on Twitter, and said: 'Just agreed with my publisher Ebury to write a book - working title "Behind the Black Door" - so chained to laptop from Monday morning.'
Brown previously ran PR consultancy Hobsbawm Macaulay Communications with Julia Hobsbawm.
She had a low public profile until she joined her husband on the campaign for the general election.
Brown is the latest in a series of high profile political figures to write about their time at the heart of power. Tony Blair's former head of communications Alastair Campbell launched the first volume of his diaries earlier this month, while Tony Blair and Lord Mandelson are publishing their diaries later this summer.