"The court reduced the sentence on Sager al-Hashash to one year in jail with immediate effect," Mohammad al-Humaidi said on his Twitter account.
The lower court sentenced Hashash to two years in prison on March 7 for insulting the oil-rich Gulf state ruler through remarks on Twitter.
He has been in jail since then and will remain there until the supreme court reviews his case.
Hashash is also on trial in a number of other cases on similar charges and is one of 70 opposition activists and former MPs being tried for storming the parliament building in November 2011.
Kuwaiti courts have over the past few months handed down prison sentences to several opposition activists and former MPs for remarks deemed offensive and insulting to the emir, Sheikh Sabah al-Ahmad al-Sabah.
Earlier in April, the lower court sentenced opposition leader and former MP Mussallam al-Barrak to five years in jail over remarks he made at a public rally in October deemed offensive to the emir.
The ruling sparked street protests by opposition activists and condemnation by international human rights groups.
Criticising the emir in Kuwait is considered a state security offence.
Those convicted face up to five years in jail.
Dozens of opposition activists gathered in the capital Kuwait City on Sunday night in solidarity with detainees but were prevented by riot police from staging a march.
Kuwait has seen many opposition-led demonstrations in protest against changes to the electoral law, which opposition groups say allowed the government to influence election results and elect a rubber-stamp assembly.
Source: AFP, via I-Net Bridge
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