Giannopoulos began his pro career with the Greek League club PAOK Thessaloniki, during the 2005–06 season.

On March 23, 2022, he parted ways with the club, after an injury-plagued second season, where he appeared in only 9 games and averaged 7.4 points, 2.1 rebounds and 1.2 assists per contest.

On April 8, 2022, despite an initial agreement with AEK Athens, Giannopoulos returned to another former club of his, Peristeri, for the rest of the season.

In 2 regular season games, he received limited playing time, while never appearing on court in the team's two playoff matches against Olympiacos.

On August 17, 2022, Giannopoulos moved to Kolossos Rodou and was named team captain due to his experience.