Fonseka, who ran against President Mahinda Rajapakse in 2010 elections, is already serving a 30-month prison term after a court martial convicted him of irregularities in military procurements.
The three-judge bench in a two-to-one judgement found Fonseka guilty of violating emergency regulations by "spreading rumours and causing public disorder" in an interview with a weekly newspaper.
He had alleged that President Mahinda Rajapakse's brother, Defence Secretary Gotabhaya Rajapakse, had ordered the execution of surrendering Tamil rebels in the final stages of Sri Lanka's separatist conflict in May 2009.
One of the judges cleared Fonseka of all three charges brought against him, court officials said.
Fonseka has the right to appeal.