Johannesburg - Vanessa Hani has defected from Agang SA to the Congress of the People, Cope said on Tuesday.
"Hani is a very dynamic young woman and we are confident she will add a lot of value to our movement," Cope general secretary Lyndall Shope-Mafole said in a statement.
He said that Hani's addition to the organisation showed the party's growing appeal and that it remained "the only viable opposition to the ruling party".
Hani, the daughter of slain SA Communist Party leader Chris Hani, said it was a difficult decision for her to leave Agang, but that Cope had always been part of her home.
"I feel I have joined a ship with a captain that has been tried and tested," said Hani.
She said that her vision was to see a democratic, accountable country but this could not be achieved by the current political leadership.
"Cope is a party that is willing and ready to work with everyone. That is one of the issues I considered before making this decision," said Hani.