The decision came as a direct response to his social media posts on Instagram during Game 4 of the championship series against Olympiacos.
Initially, the suspension for incidents during that game was announced to be six months, but the penalty was subsequently doubled.
The official ruling states that Giannakopoulos's statements severely damaged the competition's reputation and violated the Greek championship's disciplinary regulations.
In addition to the individual penalty for the owner, the club was also sanctioned. Panathinaikos will have to pay an additional 50,000 euros, bringing the total amount of fines in this case to 100,000 euros.
The decision was announced on the very day of the decisive Game 5 of the finals, further escalating tensions in the already heated rivalry between Greece's two biggest clubs.
Heading into the crucial matchup, the final series stands at 2-2, with the championship title to be decided this Saturday in the winner-take-all contest.
Ban Also for Nunn's Partner
Discontent has been simmering at Panathinaikos since their EuroLeague elimination, making a domestic title an imperative. In the domestic championship final, it wasn't just the club owner who was on edge; sparks flew everywhere, including inappropriate behavior from Blen Kiyi, Kendrick Nunn's partner.
She has been banned from all sports venues for one year. The penalty followed an incident during a tense game, decided only after two overtimes, where she grabbed referee Petros Papapetrou by the sleeve and verbally abused him.
VIDEO Dimitris Giannakopoulos Involved in Accident