The ruling means that Olmert, who entered prison in February, is due to serve a total of 27 months behind bars for the corruption case and other convictions for bribery and obstruction of justice.
He had appealed against both the conviction and sentence for receiving envelopes of cash from a US businessman while trade and industry minister in the early 2000s.
"The justices ruled unanimously to reject the appeal, both regarding the conviction and the severity of the sentence," said a transcript of today's hearing, released by the justice ministry.
He was later given an additional month on a separate charge of obstructing justice.
More From This Section
He became Israel's first former premier to serve jail time when he entered Maasiyahu Prison in the central city of Ramle on February 15.
Olmert was initially given a six-year sentence in May 2014 for taking bribes in the early 2000s in connection with the construction of Jerusalem's massive Holyland residential complex, but the sentence was later reduced to 18 months.
Olmert resigned as prime minister in September 2008 after police recommended he be indicted for graft, but he remained in office until March 2009, when Likud leader Benjamin Netanyahu was sworn in to the post, which he has held ever since.