South Australia premier Mike Rann will step down in October.
The ABC reports Mr Rann had been expected to face a leadership coup if he did not bow to pressure to stand aside.
He will leave office on 20 October. Education Minister Jay Weatherill will replace him.
The premier had previously insisted he would choose the timing of his own departure.
Mr Rann told reporters on Monday he was putting the interests of the state ahead of those of the Labor Party by leaving earlier than he intended.
He had previously planned to stay on as leader until March next year, mid-way through his government's third term.