Former MP Jamil Kazem tweeted to attack the government of the Sunni-ruled kingdom over reported bribes it paid for candidates to take part in the legislative elections.
Al-Wefaq said his conviction, including a fine, showed "the regime does not believe in political dialogue but rather in arbitrary procedures against those who do not share its opinions".
The sentencing comes as Al-Wefaq chief Sheikh Ali Salman, whose arrest on December 28 sparked clashes between protesters and security forces, remains in detention.
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Bahraini authorities have said his detention has been extended pending a probe into charges including seeking to change the regime by force.
The United States has expressed concern over Salman's detention, warning it could only inflame persistent violence that has gripped the kingdom, home to its Fifth Fleet, since 2011.
At least 89 people have been killed in clashes with security forces since month-long Shiite-led protests were crushed in 2011.
Hundreds have been arrested and put on trial, human rights groups say.