The Real Madrid and France striker was one of five people standing trial over the attempt to extort former international team-mate Valbuena in a case dating back to June 2015.
He has always denied the accusations and one of his lawyers, Antoine Vey, expressed shock after Wednesday's verdict, before confirming that an appeal will be lodged.
Benzema was not present at court in Versailles as he prepares for Real Madrid's Champions League clash with Sheriff.
Valbuena, who now plays for Greek side Olympiacos, was also not in attendance.
Benzema had been accused of helping the four other men blackmail Valbuena over an intimate video that had been taken from Valbuena's mobile phone.
The other four defendants were also found guilty. Axel Angot received two years in prison, Mustapha Zouaoui two years and six months in prison, Karim Zenati 15 months in prison and Younes Houass an 18-month suspended sentence.