Salman had been sentenced in July 2015 to four years in jail after being convicted of inciting hatred in the Sunni-ruled Gulf kingdom.
But the appeals court in May more than doubled his jail term to nine years after reversing an earlier acquittal on charges of calling for regime change by force.
Salman's arrest in December 2014 sparked protests in Shiite-majority Bahrain.
The cassation court had rejected a request to release the cleric earlier this month.
His jail sentence is part of a crackdown on the Gulf nation's largest opposition group, which has been dissolved by a court order over accusations of "harbouring terrorism".
Al-Wefaq had the largest bloc in parliament before lawmakers walked out in February 2011 in protest over a deadly crackdown on Arab Spring-inspired protests.
The crackdown on Wefaq has drawn criticism from UN chief Ban Ki-Moon and Bahrain's allies in Washington, as well as rights groups.
Hundreds of Shiites have been arrested and put on trial since security forces backed by Saudi-led troops crushed in March 2011 month-long protests that demanded democratic reforms.
Authorities have also stripped at least 261 people of their citizenship since 2012, according to the Bahrain Centre for Human Rights, including the country's Shiite spiritual leader Sheikh Isa Qassim.
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