Former Cope and ANC MP Nosimo Balindlela has joined the DA, the party's leader Helen Zille said on Tuesday.
"Today, on behalf of the DA, I am delighted to welcome Nosimo Balindlela into our ranks," Zille said in a statement.
Balindlela resigned her Congress of the People membership and her seat in the National Assembly.
She would sign a Democratic Alliance membership form and receive her membership card from DA Eastern Cape provincial leader, Athol Trollip.
Zille said she had known Balindlela for many years "as a person of integrity". It was clear "she is someone with the core values that will take this country forward".
Zille said there were many South Africans like Balindlela frustrated by present divisions, and who understood the future lay in transcending "obsolete political formations".
She described Balindlela's move as a step in the realignment of politics in South Africa, and invited others like her to join the DA.
Balindlela was an ANC-appointed premier of the Eastern Cape from 2004 to 2008 before she was removed by the party. She had heard rumours this was about to happen while on an official visit to China.
In an interview at the time, she told Sapa that ANC secretary general Gwede Mantashe said she was fired over a lack of service delivery.
Balindlela later joined the Congress of the People, formed by a group which broke away from the ANC when then president Thabo Mbeki was removed as party leader.
Cope said on Tuesday it still respected Balindlela.
"Cope recognises and respects the right of Balindlela to exercise her freedom of association and therefore, albeit regrettably, accepts that she has chosen a different, but not opposite, path of continuing her political contribution to South Africa," the party's acting chief whip Dennis Bloem said in a statement.
"We take this opportunity to thank her for her efforts towards building Cope and wish her well for the future, confident that she will continue to remain the valued daughter of South Africa we knew."
However, Cope in the Eastern Cape said her move was "a water testing exercise" by a group loyal to Cope president Mosiuoa Lekota.
Provincial spokesperson Nkosifikile Gqomo, who is part of a Cope faction that does not accept Lekota as president, said the leader was trying to reboot his career.
"The act of defying and running away from Cope... has been part of his plan to collapse Cope into the DA, so as to safeguard his waning political career," he said.
"We believe that Lekota and his sycophants, who are mostly former Nationalist Party [members] and others with blind loyalty like Nosimo Balindlela, have a lot in common with the DA."
ANC spokesperson Jackson Mthembu denied that Balindelela was a national executive committee member of the party.
"It is misleading to associate Balindlela with the ANC as she was a Cope member before joining the DA. It is typical of the DA to use cheap propaganda to create hype over nothing," he said.
"It is a known factor that Balindlela was the least effective premier in the Eastern Cape, hence she was recalled."
Balindlela is a teacher by profession who first because involved in the struggle against apartheid at the University of Fort Hare.
She was a prominent United Democratic Front activist in the 1980s, and in 1994 was elected to the Eastern Cape legislature, where she was appointed MEC for education, sports and culture.
She served on the provincial executive committee of the ANC from 1996 to 2008, the provincial working committee from 2004 to 2008, and was the provincial chairperson of the ANC Women's League from 1996 to 2001, according to a DA-supplied biography.