Thailand's Supreme Court sentenced Jatuporn Prompan over a 2009 speech accusing then-PM Abhisit Vejjajiva of being a "tyrant" and ordering the military to shoot at protesters in clashes that year.
He is "sentenced to one year in prison without suspension", despite his case being twice dismissed by lower courts, Jatuporn's team confirmed on his Facebook page.
The verdict is likely to fuel anger among the Red Shirts, a once-vociferous, pro-democracy street movement that has been silenced by the junta that seized power in 2014.
Thailand's pro-democracy supporters complain of judicial bias in favour of their rivals who are drawn from the royalist Bangkok elite and a military with a penchant for coups.
The junta has promised to hold elections in 2018, though previous poll dates have slipped.
Since the coup Jatuporn has been targeted by a number of court cases, and been forced to attend mandatory "attitude adjustment" sessions for his criticism of the regime.
Ex-PM Abhisit filed the lawsuit that has seen Jatuporn jailed.
Abhisit's government killed scores of Red Shirt protesters in 2010 as political protests left much of central Bangkok in flames.
In 2016 Abhisit was acquitted of murder charges linked to that crackdown.
On Friday Thailand's last civilian premier, Yingluck Shinawatra, is due to appear in court for what could be her final hearing over a costly rice subsidy policy.
She faces a criminal negligence charge over the policy which carries up to a decade on jail.
A civil lawsuit is seeking $1 billion in damages from Yingluck.
Source: Times of India