Brandt, 73, who had led Germany's oldest political party since 1964, quit amid a party uproar over his selection of a Greek woman and non-party member, Margarita Mathiopoulos, as the Social Democrats' chief spokeswoman.
Mathiopoulos, 30, withdrew from the appointment a few hours before Brandt's resignation, which came after a week of internal party controversy over her selection.
Brandt's resignation was announced this afternoon by deputy party chairman Johannes Rau, the Social Democrats' unsuccessful candidate for chancellor in the Jan. 25 federal election.
Rau, emerging from a meeting of the Social Democrats' national board in Bonn, said Brandt decided to resign to "cut short the unacceptable discussion over his successor."
Brandt had previously said he would step down in 1988. But pressure for a leadership change before then has mounted after a string of Social Democratic election losses in the past year and the March 16 appointment of Mathiopoulos as spokeswoman.
Mathiopoulos said earlier that she was rejecting the position as party spokesman "under the given circumstances" and that she was departing to avert further political damage to Brandt.
The Greek woman, who speaks five languages and has most recently worked in corporate public relations, was born in Bonn and recently applied for West German citizenship.