The 26-year-old, who has made 28 international appearances, spent last season on loan at Montpellier.
Sylla had two years in the English Premier League with Aston Villa between 2013-2015.
“I am happy to enter the big family of Panathinaikos, one of the most historic clubs with enthusiastic fans,” he said.
“I am here to give my best to achieve the goals set by the team.”
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