The court ruled that he lent credence to allegations the defence secretary ordered Tamil Tigers to be killed as they tried to surrender in May 2009.
Reacting to the verdict, Fonseka said that he did not accept the judgement.
He is already serving 30 months after a court martial convicted him of irregularities in army procurements.
Sarath Fonseka led Sri Lanka’s military to victory against the Tamil Tigers in 2009.
But shortly afterwards he fell out with President Mahinda Rajapaksa, challenged him in a presidential election, lost and was arrested on a variety of charges.
The court found him guilty on the charge of “spreading disaffection” – relating to a newspaper interview in which he apparently backed allegations that defence secretary Gotabhaya Rajapaksa ordered surrendering Tamil Tiger leaders to be shot.