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.