Israel faces further political uncertainty after the Labour party, a key coalition member, voted to replace its much-criticised leader.
Mr Barak's comeback follows six years in the political desert
Amir Peretz finished a distant third in Monday's leadership primary, behind former PM Ehud Barak and onetime domestic intelligence chief Ami Ayalon.
Both say they will eventually take Labour out of the government, unless Prime Minister Ehud Olmert resigns.
The two frontrunners will take part in a second round of voting on 12 June.
Mr Peretz, who also serves as defence minister, has been facing growing pressure since a government inquiry criticised his conduct of last year's war in Lebanon.
Mr Barak won the first round of voting with 36%. Relative newcomer Mr Ayalon received 31% of the vote, while Mr Peretz polled 22%.
About two-thirds of 104,000 eligible voters participated in the Labour primary.
Mr Barak, 65, says he is prepared to serve temporarily in an Olmert government, but push for early elections.
Mr Ayalon, 61, says he will take Labour out of the government coalition if Mr Olmert's Kadima party does not replace him as leader.
"I think many people understand that we are, in fact, not just voting on the future of the Labour Party but to a very large extent on the future leadership of the state of Israel," Mr Ayalon said.
During the campaign, Mr Barak has shunned the media, instead spending his time talking directly to party members.
He served as prime minister from 1999 to 2001 - two turbulent years during which Israeli troops withdrew from Lebanon, peace talks ended in failure and the Palestinian intifada, or uprising, began.
He has spent nearly six years in political exile after being crushed by Ariel Sharon in a 2001 election.