Mrs Danczuk, who found fame posting "selfies" on Twitter, was hoping to be chosen to stand for Labour in the marginal Bury North constituency.
But the party has instead chosen James Frith to stand for the seat, currently held by Conservative David Nuttall.
Mrs Danczuk has tweeted her support for Mr Frith and said she will campaign alongside him.
The mother-of-two served as a Labour councillor from 2012 to 2015 in her hometown of Rochdale.
She later became famous for posting revealing photographs on Twitter.
The 33-year-old waived her right to anonymity last year after her brother was jailed for raping her as a child.
She became patron of the National Association for People Abused in Childhood and said she wanted to become an MP to help victims of sexual abuse.
Mrs Danczuk is the ex-wife of Simon Danczuk, the sitting MP in Rochdale.
He was suspended by Labour in 2015, pending an investigation, following revelations he exchanged explicit messages with a 17-year-old girl.
Mr Danczuk, 50, who currently serves as an independent MP, recently said he "expects to fight the general election for Labour" in Rochdale on 8 June.
Karen Danczuk has failed in her bid to be selected as a candidate to become an MP at the general election on 8 June.