Olmert, the country's first former premier to serve jail time, did not speak to reporters when leaving the Maasiyahu prison in central Israel.
The 71-year-old, premier between 2006 and 2009, was convicted of graft and entered prison in February 2016. He had been sentenced to 27 months.
Olmert was granted early released by a parole board on Thursday and prosecutors decided not to appeal the decision.
He had resigned as prime minister in September 2008 after police recommended he be indicted for graft, but remained in office until March 2009, when right-wing Likud leader Benjamin Netanyahu was sworn in to the post, which he has held ever since.
Olmert won international acclaim for relaunching peace efforts with the Palestinians at the Annapolis conference in the United States in 2007, but they failed to bear fruit and the corruption charges against him have come to define his legacy.
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