Olmert has been battling a wave of corruption allegations for several months, and on July 30 he said he would step down once a new leader was chosen in the unprecedented party vote.
Several days ago Foreign Minister Tzipi Livni was elected leader of centrist Kadima party.
That was a stimulus for Olmert to resign.
"I hope that Tzipi Livni will succeed in forming a national government with the composition she wants," Olmert said in remarks broadcast on television. "For my part I will help her with all my strength."
Olmert must still submit his resignation to President Shimon Peres, who will grant Livni 42 days to form a new government and avert snap general elections, which polls indicate would bring the right-wing Likud party to power.
As foreign minister, Livni has led negotiations with the Palestinians, which were formally relaunched 10 months ago with the stated goal of ending the decades-old conflict by the end of the year.
These negotiations could complicate Livni's efforts to form a new coalition, with the ultra-Orthodox Shas Party - a key partner in Olmert's administration - having vowed to quit the government if the question of Jerusalem devision between the Israeli and the Palestinian is even discussed.
Author: Ksenia Kochneva
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Israeli PM Resigns
Prime Minister Ehud Olmert announced his resignation on Sunday