In the summer of 1999, at the age of 19, Diamantidis began his professional career, when he signed with the Greek Basket League club Iraklis Thessaloniki. In his first pro season, in the 1999–00 Greek League season, he averaged 1.5 points, 1.6 rebounds, 0.8 assists, 0.8 steals, and 0.3 blocks per game. In his first season with the club, Diamantidis also played in the European-wide secondary league, the FIBA Saporta Cup. Diamantidis averaged 1.8 points, 1.2 rebounds, 0.4 assists, and 1.2 steals per game, during the Saporta Cup 1999–00 season.