The United Democratic Movement lost its finance spokesperson on Friday when MP Martin Stephens crossed the floor to the Democratic Alliance.
He is the first UDM MP to defect since the 15-day window for MPs and MPLs to change parties without losing their seats opened on Thursday.
"We welcome him and his endorsement of the DA's vision to build a real alternative to the ANC," DA chief whip Douglas Gibson said.
"Mr Stephens brings with him a considerable wealth of financial and legal knowledge and experience. Back in the 1970s, he was the youngest member of parliament in South Africa when he won the Johannesburg constituency of Florida from the National Party.
"Under the banner of the United Party he represented the people of Florida between 1970 and 1974."
Gibson said the DA was in discussion with a number of MPs and MPLs from other parties.
"We are only prepared to accept new MPs and MPLs who share our vision of a principled opposition force which can present a real alternative to the ANC.
"Those who accept our principles, our vision for a better South Africa and our discipline are welcome; opportunists are not," Gibson said. - Sapa